TV in a junior book has to be really, really, really, really good. Otherwise the reaction will be, "Why'd you do that?"
Production quality is always going to be low. The picture's always going to be terrible – no matter what kind of camera you're able to get your hands on. There's just no comparing a $100,000 production (that’s a small one) with something you and a couple of friends did. So the idea has to rock.
I've seen a few TV spots in junior books. Only one has left an impression. It was all in the delivery. It just made me laugh. But it was accompanied by a B+ book, so we didn't hire him. Don't start working on a reel until you have an A book. (Not an A- book. The difference is huge.)
I can see TV becoming more and more common, just as a matter of competition. Some portfolio schools have the time and money to let their students dabble in it. But I think most agencies won't start looking for TV from juniors for many more years. Maybe in a decade it will become standard. And even then, if it’s going to be worth anything, it’s going to be about the idea.