I'm going to be very careful in what I say here, but no matter what I say it'll come out wrong. Bear with me.
I think AMC often crosses the line between support and marketing, but to a degree that's the nature of the beast. When someone asks about, say, a Futaba vs. JR product she's right there to provide GP's info. Is that support, or is it marketing? Some will argue it's both, but when a person is using the information to make a purchase decision I think it's marketing. That should be a no-no, to me.
I think people often get very uncomfortable with negative opinions about products, particularly in RC it seems. Everyone tends to jump to the manufacturer's defense when information about a bad experience is given out. Often that has to do with the way the experience is presented, but I think one of the reasons for these message boards is for modelers to give each other the strait dope on the products they've used, for better or worse. Somehow that idea falls into conflict with the manufacturer's reps that appear, and with the idea of site sponsors.
Where's the line? That's up to Marc, but he should be aware of how his users feel.