Er, as a soon-to-be-former MT user, I can tell you how *I* use multiple blogs and why it matters, to me at least.
First, on one of my sites, I have a "regular" blog and a fiction blog that is just for assorted creative writing, and yet a third for testing new features. (I did up a plugin for MT that allowed for custom dating.) You can see what I mean here: http://www.fantasist.net
On another, I have a blog for myself and my wife.
I plan to add several more blogs to the one site for an ongoing fiction project.
So, what's the one thing that MT has that WP hasn't got, yet? A single point of management for multiple blogs. It's nice to be able to sign in once and manage them all. Is it absolutly *neccessary*? No, but it is nice. And, since that's an upcoming feature of WP, that's what I'm waiting for to change over. It's going to be enough of an effort to convert, so I don't want to worry about upgrading to the multi-blog feature and reconsolidating my blogs.
I *eagerly* anticipate that feature!