Something tells me your mind is already made up, and this is less about the Six Nations and more about the license fee.
In any case, reasons why the BBC should keep Six Nations coverage:
1. The BBC spends £150m on acquiring sports-related rights, which covers events like Wimbledon, FA Cup, Match of the Day, Formula 1, Six Nations etc. This is an absolute drop in the bucket compared to the £3.6bn it receives through the license fee. Of course, extra costs go on putting on the event, but the rights will be the biggest expense.
2. The whole point of the BBC is to provide a varied and quality service that is value for money. £145 or whatever it currently is, ie around £13 per month, for BBC 1-4, all the radio services and the online service, is exceptional value, when you think of how much a Sky/BT package costs. People on a budget would be priced out of entertainment without the license. The license fee has stayed steady over the years, and the BBC is still an exceptional service. Rugby union is one of the most popular sports in the country so it deserves terrestrial coverage.
3. Making some rugby available on the BBC widens the coverage, making it available to more people, and widening the interest in the sport. Without Six Nations coverage, the only rugby left on terrestrial TV would be the WC, every 4 years, on ITV, and a crappy Premiership highlights package. (I watch rugby, and I never watch that highlights show because it's terrible.)
4. The BBC coverage is still the best around. I'm getting more frustrated with the crappy camera work missing lineouts and such, but overall it puts on a good show.
There's a very simple solution if you don't like it though; opt out. Before, I was concerned for the people who wanted to get Sky and didn't want to pay for the services of the BBC. But now there are some great online alternatives to television eg Netflix, Lovefilm etc. I think the BBC helps many more people have access to quality television (also bear in mind that there is original BBC programming which will bring in money on rights worldwide, eg Doctor Who). And, well, after all, if you can afford Sky, you can afford the license fee...