themes and addons can be installed on a site by site basis to answer ergophobe's concerns.
I don't think I explained this very clearly. Let's say I have two sites. Site 1 runs plugins A, B and C. Site 2 runs plugins A, B and X.
Wordpress comes out with a new version that fixes crtical security issues, but also makes some changes that require plugins to be updated. The developers for A, B and C all update their plugins, but plugin X is an abandoned project and will not run on the new version of WP.
Now I'm stuck - I have to either stick with the old version of Wordpress, update the plugin myself, find a replacement plugin, or go without. If it's critical for the one site, all sites are now held hostage and I can't upgrade the Wordpress core until I solve the plugin situation.