I'm a sucker for free t-shirts, key chains, etc etc... I have made a few purchases from AVS just to receive them. I should say I ordered them sooner than I would have otherwise. I usually wear them when working on my own stuff, not to shows or anything however.
You made it clear that you don't believe forums to be a profitable venue, and I couldn't tell you what it costs to be a sponsor on one so I really wouldn't know. I have purchased my fuel tank, pumps, and sender from a forum sponsor on pro-touring even though I could have gotten a military discount and ordered direct for the same cost. I also purchased a TKO 600 on behalf of my father from a sponsor, and have plans to purchase a Vintage Air kit and fuel injection over winter from forum sponsors. I can generally find things cheaper or same cost as the forum sponsors, but don't for a couple reasons. If they are sponsoring the forum, they make damn sure that any board members get great service. The other reason is that forums like Pro-touring, lateral-g, and this one have all taught me more than years of experience as a mechanic and dozens of books ever could, so if they are willing to support that quality of forum, I'm ready to support them.
I know not everyone shares my feelings and you have already shown your opinion, but I just thought I'd give you another insight from the "other side". I am always down for free t-shirts though, must be my Dutch heritage!