Only if on a flight - and at other times only if its a video podcast. I use my desktop for movies and my iPhone while at school. If your laptop is bigger than your TV, I say go for it, shouldn't hurt any.
ALL the time. I have a pretty big TV but it's out in the living room and it's so much more comfy to cuddle up in bed and watch a movie than to lay on the couch. It's absolutely not bad for your computer - your computer was designed to do this (and of course many other things as well). Plus, I only have a 15" so your 17" will be that much better.