Extra traffic. You will also get a number of low value links as a lot of spam sites seem to copy the wikipedia entries (if that's a benefit)
I have got my site link accepted in wiki and i get good traffic. Obviously it is very hard for getting accepted there. You will need to have rare content.
Its not hard to get on Wiki as its user based content... Whats hard is getting linked on popular pages, of course you can just add yourself but you get removed if you're not worthy.
Strangely, when I had blockthat.org running as a proxy site, it somehow got put in a wiki under proxy. I thought that was pretty cool, but traffic wise, no good, maybe a few here n there but nothing crazy.