In all likelyhood, they were playing 3.5 because their buddies were, and were on the boards for the same reasons as everyone else- to gain tips.
Look at it this way: I hate GURPS. I really hate GURPS.
But for 4 years, I played a lot of it because the game group I was in played a lot fo GURPS. We played other games as well- RIFTS, HERO, D&D, Mekton, Mechwarrior, a playtest of ACE...but GURPS probably accounted for 25% of all of the campaigns run in that group, more than any other game except maybe D&D.
And despite my dislike of the game, I played it anyway because I liked the gamers (well, most of them, anyway). Had there been a site like this for GURPS (and I had had a working modem) back then, I would have been on it trying to get tips from other GURPS players.
Now, I still buy GURPS supplements- they're among the best out there- but if asked to play a GURPS game, I'd have to ask who else would be participating- if the group wasn't cool, I wouldn't play.