YouTube LP's = advertising...
then again, I have a friend who used to just watch let's plays instead of buying games cause his computer could not run them.
Well to be fair he's kinda like a nonexistent market then. He couldn't even play the games so there's no real loss there.
(side note let's not talk about piracy since I know that's a common argument but this is a completely different subject)
A LP is in no way a perfect substitute for actually playing a game really, hell the only LP I watch (Game Grumps) I just watch for the commentary, not because the game itself is interesting. I skip entire LPs by them because they're just flat out boring.