For me, Let's Plays tend to be less about the games being played and more about the people talking over them. I still watch Minecraft videos because I like the Youtubers playing it. As I said in the NX thread, I watched about 100 hours of Giant Bomb playing Persona 4 and still ended up completing more than one version of that game myself. Did the same with their Deadly Premonition and Chrono Trigger Endurance Runs. If anything Let's Plays have pushed me towards buying games rather than made me think I'd seen it all and didn't need to bother.
I can understand the problem developers have. Not everyone is like me and still wants to buy games even if they have watched it played through already. I tend to have the opinion though that once someone has bought their game, they can do whatever they want with it and that includes putting it on Youtube for all to see.