I don't think that's a good idea. Plugins should be single-purpose or announce multiple distinct features that they provide:
Provides: facebook-login, facebook-syndication
What happens to the hypothetical plugin with a declaration like that? Would any plugin with EITHER
facebook-login deactivate it? If not, how would another plugin that handles say, 'facebook-login' and 'facebook-comments' interact with it?
If they define different "Provides" headers, say 'facebook-login' and 'facebook-syndication', they wouldn't be conflicting in the first place.
Ultimately, the situation I'm trying to describe is that each is declaring something like:
Provides: facebook-auth, facebook-syndication
Provides: facebook-auth, facebook-og, facebook-login
Users tend to want plugins to DO something, so they choose monolithic plugins over modular ones to get the marquee features. Deactivating either one of the above examples because of a conflict in
facebook-auth is likely to confuse and frustrate the end user, who doesn't know about the building blocks, and just wants both features.
That in turn will force plugin developers to either avoid the
Provides line or create unique "feature" names, sacrificing modularity. Allowing plugins to strategically disable conflicting features without deactivating other features would, on the other hand, encourage modularity and adoption.