The TV licence is the best thing that has happened to television, despite all the discourse mentioned in this node. There are many reasons for this, but the primary reason is quality.
If you have been in the USA recently, and watched the local news on just about any station, you have the presenters (big breasts and hot-looking, if they are women, or Clark Kent-replicas, if they are men) casually chatting about taking their cats to the vet, about how fast their kids are growing up, and occasionally remembering that there is the occasional news story to be presented. This situation is the result of a long and thorough process towards profitability: It is cheaper to have two geeks chat about random shit in a studio, rather than to send actual (*gasp*) journalists out in the field, to report on (*double gasp*) news stories. With all the disadvantages - not only the disadvantages to the news reporting as a whole, but also to the general dumbing down of society as a whole.
In contrast, take the UK and Norway - the two countries I am most familar with, and both countries have TV Licencing. The situation in Norway is well described by NordicFrost above, but the further implications are deeper: One of the strongest arguments against the TV licence, is that "I never watch BBC". However, there is strong evidence to suggest that even people who do not watch BBC (or other state-funded television stations) are advantaged by its existence.
The fact that BBC has high-quality programming - especially its news services, BBC Radio 4 and similar news outlets - means that the other news content providers in the UK (ITV, Granada and Channel 4 news, most notably) have something to compete against. While Channel 4 usually has a populistic slant on their news, and while ITV usually faff about too much for the news to have any deep-running quality, they always have the axe hanging over them: If the quality of the news becomes too low, people will watch the BBC news instead, which is bad for revenue, as fewer viewers means less advertising revenue etc.
Another advantage is that the BBC don't have to "defend" their spending to sponsors in the same way as, say, Channel 4. This means that they can produce series / programmes / documentaries that have a narrow field of interest, and still air them in or around prime-time. An action like this, which would be suicide to a commercial television station, makes perfect sense for BBC and other public service broadcasters; They know that competing with the populist drivel the other channels broadcast is futile: instead, they aim quality programming at the viewers who are sick and tired of that very same drivel, and want a slightly different approach.
Another reason why the licence fee is A Good Thing, is that there is only a limited amount of advertising money out there, and there is no evidence to suggest that advertisers are loyal. Advertisers know very well that the beeb has a huge amount of viewers, who will keep watching, TV licence or no TV Licence, advertising or no advertising. This means that the BBC will have less money to play with, but the beeb has one of the most thorough infrastructures of any media emporium in the UK, with relatively low running costs, compared to some of the other media production companies. If all TV channels in the UK were to go commercial, this means less money for everybody, but there is a good chance that the BBC would be able to take this punch a lot better than some of the other channels. Which, in turn, means that the BBC would still be better, but all the other channels would get even worse programming. The last thing we want.