My biggest reason for joining would probably be the taking part in focus groups etc. I really want to contribute to giving feedback on the escapist and this is probably a more scientific/sensible way to do that than, say, ranting in a forum.
Although... I'd probably feel obliged to *shudder* make friends with some of the community.
edit: also, I don't get the point of HD if they're going to mainly do cartoons, I mean who cares? Whilst I'd love to have Doomsday Arcade on my iphone I don't care for "A good knights tale" or "I hit it with my axe" which as far as I can tell are the only thing that's going to benefit... well... maybe LRR and ENN but really I don't watch that for the way it looks. If it's going to work it'll need more content like DA and "There will be brawl" which will benefit from it. I might have to create some myself... maybe enter the next film festival... we'll see...