Agreed--despite how much garbage a lot of people who wear them bring to the forum, those signature ads also incentivize the really good posters to post more often, which almost ensures that threads don't end up dead and that there's always some topic somewhere worth participating in. 

I mean, look at other crypto discussion forums (take your pick).  If you look at the first page, oftentimes the most recent post was from a day or even a month ago.  I'm afraid that if Theymos were to suddenly ban campaigns and bounties, bitcointalk would suffer a similar fate.  It might not, given how popular bitcoin is compared to even the hottest altcoin, but I can almost guarantee you traffic would drop off a cliff.

Besides, there's always the option to put signatures and avatars on ignore if you find them obnoxious.