His knock at the SCG in 10/11 was pretty important as well, although admittedly we'd already retained the Ashes.His innings at Trent Bridge last year was quite vital. It seems less important in hindsight because England smashed that game, but that was only after Bell had played that knock, before it we were struggling. In general you can say he was England's best batsman during our winning streak because of the number of different situations he got a ton from.
No doubt he's regressed hugely this year though. It's curious because before he was originally dropped in 09 he didn't seem to have a problem against spin or playing in Asia - at least he was no worse in that regard than with anything else. He was England's best batsman on the tour to Pakistan in 2005/6 and Sri Lanka in 2007/8.
Nevertheless, he's regressed significantly and he simply isn't trustworthy again. I believe he needs dropping.