Well I'd say the average mmorpg has about 2k people on multiple servers. So 10 dollars a month would be much more than clicking a banner once every few hours or more. I'll take daoc of example..it had atleast 10-15 servers ranging from 1-3k people on all of them.....sooo at the minimum 1000 people times 10 servers = a good load of money for them to update and maintain customer/ingame support. Besides, I'm sure people would click the banner once and leave the game open forever or make a bot for it so you wouldn't get kicked off by some idle timer.