Leave it to Alan Moore to succinctly explain (way back in the 1980s) the major problem I see with continuity-altering events like Crisis on Infinite Earths and One More Day:

I believe this is dangerous for a couple of reasons. Firstly, by establishing the precedent of altering time, you are establishing an unconscious context for all stories that take place in the future, as well as for those which took place (or rather didn’t take place) in the past. The readers of long standing, somewhere along the line, are going to have some slight feeling that all the stories that they followed avidly during their years of involvement with the book have been in some way invalidated, that all those countless plotlines weren’t leading to anything more than what is in some respects an arbitrary cut-off point. By extension, the readers of today might well be left with the sensation that the stories they are currently reading are of less significance or moment because, after all, at some point ten years in the future some comic book omnipotent, be it an editor or the Spectre, can go back in time and erase the whole slate, ready to start again.

Moore goes on to discuss whether or not throwing continuity to the wind is a good or bad thing, and weighs the pros and cons of both, but I was struck by how much the above paragraph is relevant to what’s going on in superhero comics these days.