Here’s what I think it’s important to think about when deploying a Wiki/Blog environment:
I think Wiki/Blog (let’s just say self-service publishing) can be very powerful where processes for publishing “formal” information channels to the Intranet are in place. That is to say, if the right people are publishing to the right place on the Intranet and have the right editorial flows/governance around that, then adding an open, less-structured layer to that is fine. If, on the other hand, if there are no good controls/process/tools in place, then handing everyone a Wiki is too loose a solution, in most cases.
Certainly there are models of total self-publishing which have worked well. Rational, for example, followed that model but this was a particularly techy and tight corporate culture (at least, before IBM acquired it ;) ). So overall, my advice would be to make sure they have their formal content publishing ducks in a row before introducing self-publishing, and I certainly would not advocate replacing a full-blown CMS with a Wiki.