Theoretically, if you have a (T)Wiki, you shouldn’t need a blog, because if you have something to publish internally you could just create a page on the Wiki, right? Or if you have mailing lists available, you shouldn’t need forums should you?
In practice, though, I find that having or not having a tool that feels natural to use affects how often I actually go create a post. Examples: musings on designs, thoughts about ways we might want to solve certain problems in the future, things we figure out with the technology… Twiki pages feel more formal somehow, and a blog feels better suited to jotting down quick thoughts.
You can disagree about individual perceptions. For instance, you might say, “Twiki is fine for half-baked thoughts and non-thoroughly-vetted ideas!” — but if a potential author feels a mismatch between the message to publish and the medium through which to publish it, it can be a barrier to creating that post. (It’s hard to measure how many posts don’t get created because of such friction, but I contend that the issue exists! I think it’s sort of a pies-aren’t-the-same-size thing.)
The important thing to realize here is that you can save work by blogging. Many people have figured out that this is true of Wiki: rather than explaining the same thing over and over, you put your explanation in Wiki once, and you’re done. From then on you can just point people to it.
But Wiki doesn’t feel like the right medium for the kinds of things that go into blogs. Blogs are often more spontaneous, more exploratory, and people have different expectations about them. That’s not to say Wiki isn’t wonderful — it is. And there’s certainly some overlap between what people put in the two, because blogs and wikis are (today) the primary mechanisms for persistent communication, by virtue of having the lowest friction. Given that we have only these two models for self-publishing today, both of them are going to be stretched a bit as people try to figure out how to make them work for N types of communication. But for some things you write, your blog is clearly the right venue.