Indeed if he is so rubbish yet he still has outqualified all his team-mates.
Indeed if he is so rubbish yet the RedBull has a decent car. Many people forget the design the following years car happens around April/May of the previous year.
Indeed if he is so rubbish Mario wouldn't have asked him to go to BMW.
Indeed if he is so rubbish, then Toyota/Renault/Williams wouldn't have bidded for him the year before.
He came to Williams expecting a fast car (they won the last race of the season) yet when he turned up there they had wind tunnel calibration problems, poor traction control, an crap engine that needed rev limiting both in high and lower temperatures, a car that was crap on its tires early on, a car that was stiff and crap over the kerbs. And this is Williams, he had higher expectations than driving in the midfield again because they were Williams. And now that the car has been @#$%& and they have no manufacturer backing, he has every right to walk away because he expected more (and vice versa). He has had a disappointing season by his standards because normally he doesn't make so many mistakes. Watch his season both at Jaguar and Minardi - he made more mistakes this season than all 3 seasons put together at those teams.
Fans are as fickle as the sport comes when it comes to perceptions. If he wins a few races next year in a competitive enough car, everyone will be lauding him a champion. If you go to a team like Mclaren, Williams, or Ferrari you expect they have the right infrastructure and right people to be competitive and be at the front majority of the time. Williams on the other hand have gone backwards.
Fans laud how good drivers are technically etc. etc. etc. but the design team are the ones ultimately who create the car, not the driver. That is why Horner says Newey is bigger than Schumacher...because he is. He understands how to make a car go quick. Any competent driver would be able to win in a front running car with a reasonable gap. I'd prefer drivers like Webber/Raikkonen/Alonso/Button anyday over pay drivers...not to mention that Webber got where he is on merit, not on $$$.