Why bother with blogging when you use a wiki?
Blogging is useful for deliberately choosing to discuss X, rather than using it as a default means of transcribing your thoughts (wiki can serve as the repository for that stuff).
Some contrast with Wiki
I think that prior to using a Wiki, I used blogs and microblogs as the sounding-boards of any/all of my thoughts, which is sorta socially negligent and a blog medium is pretty poor for allowing one (me) to really refine their thoughts.
- Good for people who've needed to keep their mouthes shut, especially if they're in hostile territory (or company)