I learned my lesson after Nintendo went berzerk and requested takedowns for gameplay videos and walkthroughs; smashrockgroin's series for Goldeneye 64 was both funny and informative when playing that game, but now those videos are gone forever. I've vowed never to buy a Nintendo product again because of that alone.
Going after pirates and such I can sort of understand, but people enjoying your product (and obviously helping to promote it)? WTF.
It reminds me of the complete opposite of this old meme: http://lurkmore.so/images/9/94/360_Kid.jpg
The usual rationale is that walkthroughs / let's play videos show enough of the game that people may opt to just watch the video, instead of playing the game itself.
I can sort of understand this position when it comes to videos that e.g. show the entire single-player campaign with zero commentary. But I'm not convinced that people opting for such videos would have bought the game if the walkthroughs were not available.
also, if the game has nothing else to offer when compared to a video playthrough, may be it's the game that has the problem!