It's a way of funding them as good as any other. Personally I prefer it: I don't watch television so from my point of view the budget of the public TV channels should be as small as possible.
i don't go to the library often because i buy books, but i don't think the library should be ad funded, I'm happy to pay for it in case someone else needs it, their knowledge will benefit me later anyway
That seems like a dishonest comparison. While it's true public television has some divulgation programmes, they are usually pushed to the secondary channels and they aren't watched by many people. If public television consisted only of those programmes, its budget would be much, much lower, and I would be happy to pay for it.
The truth is that public TV nowadays contains lots of series and movies (in many cases, American), cooking programmes, tabloid celebrity stuff, etc that they think they must do to stay competitive with private TV. And that's rubbish.
TV is about as far away from beneficial to an individual or society as you can get without straight up poisoning or stabbing people. The less it is consumed, the better. You're not helping anyone or society by encouraging others to watch it.
