i think the l4d server is the only one where bots would be good. zombie bots would actually make sense. Also right now the server is pretty much empty with a few exceptions(me being one) that shows up often. with a few bots it would let the first 1-2 people that get on be able to play and then others that show up would stay instead of leaving after seeing only a couple of people on.
If you do decide to add bots in the L4D server i would suggest only between 2-4 bots.
I hope this suggestion goes through. i like the L4D mode. Although i didn't like getting bumped from lvl25 with extra exp and ammo to lvl13 when the server addresses changed
