I watch them. In fact, I'm a YT LPer myself. Did my debut with Castlevania SOTN.
I'd have to say my favorite LPer is Toegoff
There are many reasons to watch LPs:
~As walkthroughs that are slightly better than the boring professional stuff
~As a COMPLETE review to see if it is actually enjoyable, even if you are just watching a random episode in the middle
~To see what the commentator thinks about the game (sometimes funny to see)
~A game is too expensive/scary/fast or slow-paced/frustratingly difficult but you still have a cat-killing curiosity burning into your soul
Blind LPs are especially good (for those who don't know, its an LP during the commentator's first playthrough). I loved seeing Toegoff's reactions through Symphony of the Night as he went through territory that was familiar to me but not so much to him. Also, I saw his blind LP of I Wanna Be The Guy and couldn't stop laughing as he played through one of the most frustrating (but possible) games to come to existence, even though I hadn't played it myself. I guess watching their misery is part of the fun?
There are so many reasons to watch LPs. The list goes on. Most people blatantly look at LPs at just watching some other guy play the game. Perhaps they haven't seen the right LP, or even tried one at all. Who knows, but personally I believe there is an LP for everyone.
Now excuse me while I go find Let's Play My Little Pony - Scuba Diving Edition: War Against Seamonkies Episode 1.