Does everyone who practices guitar ends up a great musician? And if they don't, but only practice guitar to relax, would you consider the time "wasted"?
I guess you could approach this kind of question from a risk/probability point of view. Not everyone who practices guitar becomes great at it, but some do, so if you practice guitar you expose yourself to a potentially big upside (assuming you actually want to be a great guitar player).
I can't think of a potential upside that you expose yourself to by watching TV (other than the immediate gratification), but maybe there is something.
He could have replaced guitar with model airplane building, riding bikes, making friendship bracelets, or training a giraffe.
The point is, good time management makes time for the things you need to make time for - and in this person's case, TV happened to be his vice.