Since we're talking about a niche-specific TGP here, I'll suppose there is a "limited" number of people that's into what you offer. In that case, it *could* be a good idea to keep a community - small groups tend to want to stick together. I've seen communities inside some paysites members areas that were actually successful. Still, I can't say it would work on TGPs for all the reasons you guys already listed.
However, I came across this posted on another board:
http://www.call-kelly.com/forum/
Now Call-Kelly is a big TGP as far as I know and they got a forum that's crowded with people talking and having fun. So what do you think about it? Is it possible to have an active board/community on a TGP or that's just an exception?