We got the login server for Windfola down on Lord of the Rings Online yesterday during Smooth McGroove's stream since we got dozens if not hundreds of people to try and download the game + create Lv. 1 Hobbits on that server all at the same time.
(And the point wasn't to purposefully try to bring it down, it's because his stream made everyone want to get the game, and chances are several of them got interested in the game and continued playing afterwards.)
And Nintendo would probably never have seen my money for MK7 if it wasn't for his stream.
I'll grant you that a lot of people would settle for watching someone play through a game in order to experience the story/etc. (what I did with Deadpool) when they would normally have bought it and played it themselves, but acting like every LPer or Streamer has that effect is just... no...
Then again, the only appeal of Deadpool is the jokes in it; gameplay and storyline are fairly generic, but like Veho said that's a problem with the game itself, not with LPers