Some people like the deault layout of vBulletin but not the administrative features. Some people like phpBB because it comes with LOTS of features by default that saves people modifying it.
The reason I like Simple Machines Forums is because its just as the name suggests. Its Simple to use and customize.
The best thing in my mind about the new SMF 2.0 is the custom install of modifications to other themes you have installed. It save you hours of manually having to edit your forums files. Just have to upload and select install. Sounds pretty simple to me.
Yeah, thats a really great feature in 2.0. A lot of stuff was added to the sources directory. I believe all forum systems have their perks.
Indeed; speaking as a modder, I find 2.0 a lot easier to code mods for in terms of reliability; I'm not fighting custom themes so much if I want to do anything interesting.