Let me preface this by saying that I'm absolutely notorious for giving out free trial codes. You want to check out WoW and see if you like it? Sure! Let me get you a code -- and get myself 30 days free in the process if you sign up. You think your cousin might dig the game since he plays EverQuest, but you're not sure? Here, let me email you a code to forward to them. For you see, I am every inch the devoted WoW fangirl, and I say that without any shame. As such, it is my mission to share the fun!
Now, with that said, I think I've easily managed to score four or five months of free time from people converting trial accounts to paid accounts. And as WoW gains even more ground, even hardcore devotees to other MMOs are starting to be curious about what all the noise is about. (Well, okay, maybe that's just my older brother who has been staunchly refusing all my free codes and continuing to play City of Heroes/Villains... But I sense his will starting to falter now that he's hit endgame there. Mwahaha!)
Of course, if you listen to Terrorantula on the (European) WoW General forums, trial accounts are everything that is bad with the game. They bring in gold spam mails! They let those advertisers come on and harass us! Now personally, while I'm all for Blizzard writing in some kind of process that immediately bans any account who sends certain gold-selling domain names in chat or mail (and they know who they are) I don't know that I would back the total removal of all free trial accounts.