That mantra is asinine not because YouTube (and Twitter, and Facebook a couple years ago) are actually worthless... it's asinine because they are crazy exceptions.
YouTube gets about 2 billion views a day. Assuming each one of those views lasts about a minute, that's 1.4 million people watching at any given moment, 24 hours a day. The major TV networks get in the neighborhood of 6 million viewers during prime time... if they're winning.
So YouTube has roughly the same daily attention on it as NBC does. Why aren't they profitable?
a) They've had skyrocketing costs as they deal with explosive growth
b) They've had to convince the world that this totally weird new thing is a viable advertising platform.
That they've more or less solved those problems in 4 years and are close to profitable is pretty amazing to me, both as a developer and an entrepreneur. And the fact that their costs are going to go down (cost of bandwidth is cut in half every 18 months) while their profits are rising (advertising is advertising, and they're selling more and more), along with their enormous audience, suggests to me that they are, in fact worth a lot of money.
Do you not think that you could easily sell YouTube today for 1.5b? To any of a number of media companies? I think it'd be an easier sell today than it was when Google bought them.