It also helps to sell more comics. Although readers may not exactly be excited by something called a Vibranium Plague, they just might be excited to learn that the super-cool Black Panther has his own comic book! Inter-title continuity was something that Stan Lee and later Roy the Boy practically pioneered. In many ways, worldbuilding makes for company building. Only when continuity begins to feed on itself and fan-writers become obsessed with chasing down every "what if.." and obscure corner of the universe does it translate into a decline in sales, it seems to me.
The capacity for cross-promotion is one of the benefits of this approach, but it is a beneficial side-effect and not the motive.
And my problem is, if ANYTHING, writers are more reluctant than ever to explore strange or disused corners of the Marvel or DC Universes, no doubt because of the very shallow knowledge of many writers about the books that they're writing.
Almost nobody has made mention in recent times of the fact that (for instance), Hawkeye wears a hearing aid because biting down on a sonic arrow made him partially deaf. Something this important can't go for YEARS without being mentioned.
Contrast that with all the use that was made with Simon Williams Wonder Man through the years, from his brain recording, return in the Legion of the Unliving, the Grim Reaper, and ultimately his resurrection under Gerry Conway? All this from a guy that BARELY showed up in a single early issue of the Avengers, where it should be noted, he DIED.
When was the last time the DC Universe, for instance, made use of...of all people, an A-lister like Amazo? Tom Peyer's HOURMAN, and a single issue in the Morrison JLA.
(Someone that reads more comics than I do may chime in to correct me here, but my point still stands. Someone like Amazo ought to at least show up once every two years.)
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