I personally do let my DD (now 20m) watch TV. I believe that, like most things, if "used" appropriately it can be as good as any learning tool. That's why schools and colleges use TV as learning resources.
My daughter is very sociable, has an excellent vocab and strings sentences together well, she sings and dances, she is starting to recognise colours, she can play happily with toys and books for ages and ages on her own and with others, etc. BUT she does watch Tweenies, Fimnbles, etc. and she enjoys them. She also loves nature programmes for the animals.
However, again like everything else, if you just leave a child on their own in front of the Tv and don't monitor anything being watched - yes, you may get problems.