Continuity is a much-maligned word. It's become synonymous with eternal, formulaic committee-written crossovers, and, perhaps worse, nit-picking obsessiveness about incredibly trivial pieces of comics history. It's seen as burying new ideas under the weight of forty years of fossilised data. Marvel set up a whole new line just to get away from it.

But the term is widely misused. Continuity, in the strict sense, is a good thing. It is highly desirable that stories should not contain internal contradictions. They confuse readers and draw attention away from the story. That doesn't mean it's the number one priority that should be protected come what may. But, all things being equal, continuity ought to be a positive factor. The real cause of problems isn't continuity itself, but the way in which it's been allowed to become a driving force in its own right, rather than just a handy background feature that's worth maintaining. And these are particularly acute difficulties in superhero comics, because of the way they're set up.

More through accident than design, superhero comics have ended up making heavy use of some storytelling techniques that are comparatively rare elsewhere. And high up on that list is the shared universe. It's not unknown - Kevin Smith does it, Isaac Asimov did it, a few spin-offs here and there do it - but there are few other places where it's ended up being quite such a prominent part of the story as in superhero comics.

And this is specifically a feature of superhero comics. Superhero stories in other media tend to steer well clear of the whole thing. When a superhero comic is adapted for another medium, the shared universe elements are usually the first thing to go - although often, legal reasons would compel that, whatever the creators wanted.

Contradictions confuse readers and draw attention away from the story. It's tempting to suggest that this is yet another indication of how superhero comics have gone seriously awry. Why is nobody else doing shared universe stories? Probably because no-one in their right mind would want to.

But despite the knee-jerk appeal of this theory, it's not that straightforward. Comics are uniquely placed to put the shared universe at the centre of their stories, because of their publishing format - a whole load of indefinitely continuing monthly series under the common control of a publisher who's in a position to exercise some kind of overall editorial control over the universe.

That allows the shared universe - as communal setting - to be turned into something consistent and central to the line. Realistically, with the exception of a handful of spin-off shows that retain close links with their parent, most other media are not in a position to achieve that kind of integration. Those TV shows which share a universe tend to use it more as a running joke gimmick than anything else (ALLY MCBEAL/THE PRACTICE; FRIENDS/MAD ABOUT YOU).

The appeal of the idea is fairly obvious. It gives you a ready-made mythos and backdrop in which to place a new series. Any story setting itself in the Marvel or DC Universes gets to draw from a whole range of pre-established concepts when the story calls for them, without having to re-establish them from scratch. Done right, you should end up with all of the stories contributing to the shared mythology and result in a setting that is richer than you could easily have established in a single series. It's a nice idea. It's the reason why books like ALIAS want to join in. But it doesn't always work out that way. While the idea of a shared universe is strong, it's still just an idea for a backdrop, and a device for sharing concepts. What made it a liability was the phase in which there was a painful overemphasis on continuity. That tried to turn the backdrop into a story in itself.

Comics are uniquely placed to put the shared universe at the centre of their stories. One of the points where superhero comics went wrong is when a generation of fans became creators and started playing around with the existing ideas, trying to draw convoluted connections between them, just for the hell of doing so. The Marvel Universe is indeed an appealing place. Up to a certain point, it can be amusing to draw connections between previously unrelated characters, and have major plots from one story spill over with consequences in other stories. Up to a point.

Beyond that point, you get stories that read like historical treatises on the canonical status of 1940s comics, and perfectly good stories being derailed and interrupted by crossover plot elements that are nothing more than extraneous distractions.

At its worst, you end up with a situation where the shared universe backdrop itself becomes the story, and the individual series are relegated to bit parts in the overall storyline. In the long run, this is dreadful news, both artistically (because it results in rubbish like ZERO HOUR) and commercially (because it results in readers who are trained to be interested in massive continuity-impacting events and don't see why they should be interested in free-standing series that don't impact on anything else). Needless to say, like a great many bad ideas, it was very popular in the 1990s.

Attempting to resolve everything into one absolutely consistent continuity can be very entertaining, if you have a certain mindset. There's nothing wrong with that. It's entertaining in much the same way that doing a crossword is entertaining. But when it becomes the driving force behind the stories in its own right, you end up with stories written by committee, because everything has to be co-ordinated. In any event, the Marvel and DC universes are both fundamentally unsuited to be put in the spotlight. They don't stand up to scrutiny. They belong in the background where their internal inconsistencies can be glossed over. The more you emphasise the detail, the more you invite the audience to see the joins.

To that extent, the current backlash against that approach is to be welcomed. Tight intra-line continuity is going badly out of fashion. Marvel seems to have abandoned it; DC needs to go an awful lot further, principally by abolishing its over-reliance on line-wide crossovers. The trend, nonetheless, is in favour of creator-driven, continuity-light comics, even in the superhero mainstream.

But it's important to remember what the backlash is reacting against - not continuity itself, but the emphasis that had come to be placed on continuity. Continuity, in and of itself, is still a good thing. And while it shouldn't be driving stories, it's still worth preserving.

The Marvel and DC universes don't stand up to scrutiny. They belong in the background. Slavish adherence to the minutiae of past continuity would be excessive. But there are sound storytelling reasons for trying to get it right where possible. If you're writing a story which claims to be set in the usual continuity, and it contains glaring contradictions with significant parts of established history, then that's going to leap out at the readers. At best it'll distract them, at worst they'll assume it's a red herring and spend a large part of the story wondering how the story is going to explain away this false information, instead of paying attention to the plot. This is in nobody's interests.

Equally, there is a certain category of story that just doesn't work in a shared universe. The Batman "No Man's Land" storyline had great difficulties persuading anyone that the rest of the DC Universe, as established, would put up with something as absurd as Gotham City being kicked out of the USA. The criticisms on that ground weren't nitpicks, they were fundamental credibility points. Once the story had set itself in the DC Universe, readers were entitled to hold it to the ground rules.

The recent obliteration of Washington DC in AVENGERS - subject to the possibility of a time travel cop-out - is another example. Several months down the line, there's been no reflection of it at all in any other Marvel title. That sits uneasily with the shared universe concept. God knows we don't want to go back to the days when comics were clogged up reacting to other comics' plots, but there are some story ideas where the scale demands that they either affect the whole line, or not happen at all. And this is one of them.

"Not happen at all", of course, isn't strictly accurate. There's a third option, which is to do the story anyway but not set it in continuity. This is usually the most sensible way to avoid the problem, rather than hammering a story into a shared universe when it doesn't fit.

The de-emphasising of continuity in the last couple of years was not merely welcome, it was imperative. But as a framework in which a range of series can co-exist, the shared universes can still serve a useful function. Too much of a swing in the direction of disregarding continuity runs the risk of throwing that away for no particular gain. For certain types of story, it's a useful device. Once the nit-picking obsession with detail has been stripped away, what remains is still worth protecting.

Paul O'Brien is the author of the weekly X-AXIS comics review.

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