This is where "state of the art" is really an apt phrase. Rails 3 is effectively a community consensus on the best way to make most webapps today. Rails 2 was the community's best stab at that same goal a few years ago. The distance between the two is explained by a changing state of art, and some ferreting out of weaknesses in the framework.
I think rails developers blindly upgrade their apps, just because they want to be on the hottest new technology. I know I am drawn by the glitter of rails 3.1 beta. But everyone should really weigh the business decision of upgrading; what do you really truly gain by being on the latest rails for all your apps? Sometimes the technical debt you take on by maintaining an old framework is less than the effort you have to spend to upgrade, especially for sunsetting apps.
I think rails developers blindly upgrade their apps, just because they want to be on the hottest new technology. I know I am drawn by the glitter of rails 3.1 beta. But everyone should really weigh the business decision of upgrading; what do you really truly gain by being on the latest rails for all your apps? Sometimes the technical debt you take on by maintaining an old framework is less than the effort you have to spend to upgrade, especially for sunsetting apps.