I let Oscar watch TV, somedays more than others but only dvds so he gets no adverts etc (we don't actually have normal tv, just a screen and dvd player).....anyway, he is lively, bright, communicates well and will thoroughly concentrate on other things like, books, toys, exploring outdoors etc. All in all I feel he is a healthy well balanced child. Sometimes he gets fussy and wants certain things on and gets upset when it is time to turn them off, but I've learned to make a thing of saying 'bye bye postman pat' or whatever and he seems to accept that well.
Lately, he is crazy about Thomas the tank engine... he discovered this quite by accident (long story) and now he wants 'toot toot' every day. It's a bit maddening and I don't actually like the show at all, it's really weird, but I talk to him about it and he has had a LOT of fun exploring everything train related that we can show him, so in other ways it's been really good too.
If it weren't for TV watching, I would get nothing done, and I'd be a bit of a wreck TBH.
I'd worry if he was showing signs of being badly effected by his enjoyment of TV watching, but thus far, I can only detect that it has given him enjoyment and information and besides, its part of his technological world (he also loves playing on his daddy's ipad although this is a rare treat).
I'm pretty sure the 'no TV under 3' is the simple shortcut way of ensuring that children don't get into unhealthy habits, which as parents it is our jobs to make sure they don't.
Perhaps there is something I don't know about it all, and I am constantly questioning myself about how much and when we use TV to occupy Oscar, but for now, I trust myself that we are keeping a healthy balance.