What I fear is that the multiblog version is doing two things instead of just one. They're doing multi-blog _and_ smarty template integration. I think doing the smarty thing may be at odds with the main branch's "smaller faster better" ideals, and therefore may have a hard time finding it's way into the main distribution. Maybe I'm way off.
The big problem I have is that apache says mod_rewrite is on, but wordpress MU doesn't agree.. so I can't even evaluate it. :( Very sad. This may also be at odds with the "works almost everywhere, setup in 2 steps" ideals of the main branch. The beauty of WP is how easy it is to get going. The MU branch is just more technically challenging to get going...
if they can make it work without mod rewrite and without smarty, I think there'd be little reason to keep it out of the main branch. Again, I may be way off, but that's how it looks from here.