Publishers Weekly had a piece on "Big-Cross Over Events and the Barrier Method" that sums up the situation pretty nicely. The writer Todd Allen puts most of the blame on the business end, on a need for steady sales because these are publicly traded companies. That must be true to some degree though as outsiders we will never know. Certainly the editorial people at both companies claim there's no such pressure but the amount of cancellations, relaunches, reboots and yes cross-overs shows that something other than purely creative considerations are at work.
Many writers and artists complain about cross-overs but they tend to be the less powerful ones, people like Peter David or Christopher Priest. (Grant Morrison did at one time but he's top of the heap at the moment.) The ones like Johns or Bendis that drive these events rarely say anything negative about them. In fact it was while reading Morrrison's run on Animal Man that I first really saw the problem. When going through that series, the spots where it tied in to big events at the time no longer matter and in fact only hurt the story. Morrison talks about that briefly in one of the tpbs but it's easy to see in many places.
Of course nearly all these people learn to be follow company lines pretty early in their career and we generally have little idea what they're really thinking unless something comes out later. But it looks like many creators get caught up in the idea of a Big Event and are able to convince themselves that this is actually good storytelling. And I can't entirely blame them - if DC came to me and said, hey want to write a huge story that can use any character we've ever published and make any changes you want, then heck I'd likely do it too.
(I have to add one example that also ties back to an earlier post came in a recent issue of Siege. Now this should have been a good event. The idea is simple - Osborn and his government-sanctioned supervillains-pretending-to-be-good-guys invade Asgard and everybody else tries to stop them. Four issues, potentially tight & restricted story, big cast, big stakes - easy right? Well Marvel first started advertising that "an Avenger will die!" as if that matters at all, though considering that there are now four separate Avengers teams running around maybe a little thinning of the herd can't hurt. But the death (Ares in case anybody cares but really nobody does) was a two-page spread of him being literally torn apart in very graphic detail. At some point it's hard not to think these creators really are stuck at age 12.)
One solution is what Alan Moore proposed in the never-realized "Twilight of the Superheroes" where there's a main story in a separate series and then everybody else can tie-in if they think it's worth doing. He wrote then that such an event "would have a sensible and logical reason for crossing over with other titles, so that the readers who were prompted to try a new title as a result of the crossover or vice versa didn't feel cheated by some tenuous linkage of storylines that was at best spurious and at worst nonexistent." That nails the problem we're seeing today and Moore wrote that a quarter of a century ago.
The Big Two at times appear to trying something like this by putting the event into its own mini-series and then adding other minis rather than using ongoing series. Final Crisis possibly came closest to this idea though considering the links between the minis and then to Batman: RIP in those regular books it was hardly a streamlined approach.
Another solution frequently proposed (and that even the editorial people say they're doing but never really follow through, as Allen's article points out) is to just put a moratorium on cross-overs. This seems unlikely considering that the business has adopted this blockbuster mentality - it's pretty easy to mark the decline of American film back to studio reactions to Star Wars and there's a chance we're seeing the same thing in comics.
Because what I'm really getting at isn't the events or that they might cause problems for retrospective reading - it's that so many mainstream comics creators are producing inferior work and appear to not care about stories or characters or ideas, in short to not care about all the things that drive people to create art. And the real problem is that very few of them seem to realize that.