well, piracy exist because people don't want to pay for games, right? An intelligently setup ad supported system could, theoretically, enable those who want to get the game for free could do so, direct from the developer, all the while supporting them at the same time.
Then if there was a purchasable version it wouldn't need as severe protection because the only people who would pirate it would be people that purposefully don;t want to support the developers.
I know that I for one would rather put up with a few adds than pirate a game, if it was down to those options.