Well, I suppose there's a number of reasons why you might want to be selective about what you watch on TV, though it also depends on age. Since you're a teacher, I assume you're talking about kids and teens here, so I'll give my personal opinion on that.
Television can, believe it or not, be good for the brain in some cases, but most often than not it's just a (pleasant) waste of time, because you usually don't learn anything from it or gain anything from it other than entertainment.
I do believe that being entertained regularly is necessary for a healthy and happy lifestyle, but that doesn't mean people should depend on TV alone for that entertainment -- they need to also go out regularly and spend time with their family and friends.
To sum it up, TV can be good if it's done in moderation, which I believe is an important life lesson that needs to be learned early for kids and teens.