I've heard this my entire life. But, you know... I think I've gained a lot from TV.
First of all, there's the music. All the time, I'll be watching one of my shows, and something will be playing in the background that I really, really like. So, after the show's over, I'll tap into one of my online resources for TV music, and I'll find out what the song is so I can listen to it again. It's extended my music taste so, so much. Supernatural is especially guilty of this, as it has caused me to appreciate--and actually listen to--classic rock. Smallville also introduced to me one of my (now) favorite songs of all time: "Impromptu in G flat" by Schubert. I even loved a song that was performed on House, M.D. so much that I printed the sheet music and learned it on the piano. I still know it by heart.
Believe it or not, I actually learn some history, and not just because I occasionally watch the Discovery and History channels. Supernatural has episodes based on lore and myths originating in the United States. They have plenty of stuff from other countries, too, but it's cool to learn a piece of American Heritage that you're not necessarily going to learn in a book. I actually learned about the lost colony of Roanoake because of an episode entitled "Croatoan," in which they heavily reference Roanoake. I was so intrigued, I looked it up and read everything I could find on it.
I was quite the sheltered child. That's not a bad thing, and I don't think it was intentional. I just wasn't exposed to a lot of the things everyone else was. So people will make references in their daily language to something and I'll have absolutely no idea what they're talking about. Just common phrases, all over the place. "Oh, cry me a river." "Who'd you expect, Scully and Mulder?" "Who ya gonna call?" I was lost all the time. But, as I've been watching more television, these things have been explained to me. I can participate in so many more conversations! I understand so much more. And, I can make the references myself. 
Sarcasm! I can't tell you how often I used to be fooled by sarcasm. I did not understand it at all. I thought people were being serious or mean, and used to think, "Why can't people just say what they're actually thinking?!" I got so confused. It was even worse when I tried to use it. But now, it's not so lost on me. Sometimes it still takes me for a spin, but I can understand it. That's valuable to me... It makes me not such a social pariah.
Vocabulary is also a big one. Yes, I've gained most of my vocabulary from reading, but there've been a lot of times that I've used or pronounced a word incorrectly because the only time I'd ever encountered it was in writing. I've gained words I didn't know could be used in certain ways. I've gained ways of speaking, as well as a knowledge of using tones in my voice. Now, I don't sound like such a know-it-all ALL the time. I know how to say things in just that certain, specific way, where it can be taken how I want it to. A lot of that came from books, too... But TV has definitely helped.
I've learned about people. I've been able to see personalities and occupations and attitudes and types of people that I don't see in my life. It helps me understand why sometimes people make the kinds of decisions they make. I can see thought processes that aren't similar to mine.
Television has inspired me to write. In middle school, I wrote fanfiction after fanfiction, or Teen Titans, Justice League, Batman, Gargoyles, Kim Possible, if I watched it, I had a fanfiction for it. Then, one day, as I thought about my writing (which I didn't ever actually let anyone read, for embarrassment), I suddenly thought, "I want to write something I can share with other people, that is my own story, completely made up by me." Thus Leah was born, the story that I've been writing for almost half my life, that has kept me going at some times, and has helped me to really improve my writing. I also am able to use things in my writing that I've seen on TV, and never actually experienced. I can write about a pair of really close siblings, because I've seen it.So, most of what I've learned has to do with social skills, but that's very valuable to me. I wouldn't have learned them any other way. I don't observe the behaviors of others. That's just not how my brain works. But I'm starting to get there.
You know, I do have to admit that I don't read as much as I used to. I'm not totally up-to-date with world news. But, it's nice to be able to share something I enjoy with people I love. I can sit down every day at two o'clock and watch General Hospital with my mom, and then argue over whether Sonny or Jason is better. I can quote Supernatural for hours with Jessica, laughing so hard we get to the point of wheezing. It also gives me something I can share with people that I would otherwise have absolutely nothing in common with. So, I think that in a lot of ways, TV is a great thing.
But, too much of a good thing is a bad thing. I've just got to figure out where to draw the line.
