You are correct. MT is a nice system, used by many "grown up" sites; but it goes against a couple of my requirements. Specifically:
I also could care less about multiple sites. If we want to do that, then I would certainly not use WP, and would probably go for a CGI-based system like MT.
MT also suffers from the same problem as WP: It's a blogging platform; not a portal/CMS system. The application I have in mind is basically more of a "mini-portal" than a blog. WP is being slightly bent to fit this need.
I'm also not familiar with MT's security model. Since it is a blogging platform, I'll lay odds it's similar to WP. A portal CMS would probably have a security model more to my liking. If I remember, PHP-Nuke had a security model like the one I need, but it violates almost every one of my other requirements. I had to basically rewrite PHP-Nuke once in order to produce valid code, and it was hacked by an old exploit. I never want to go through that again.