While I agree that you're probably going to be upgrading in part to amortize your costs, it's not like you're getting no value out of it. The upgrade from 4.1 to 4.2 had significant speed and stability gains in active record. And recent versions of a lot of popular gems are both faster and more secure.
Don't get me wrong, there are reasons to upgrade. But all other things being equal, you may very well not need the newer features. Rails has an astoundingly good track record with backporting security fixes, going so far as to provide monkeypatches for those that can't upgrade their gems for one reason or another.
Upgrading Rails can force upgrades on a lot of other things, however. And new bugs get introduced all the time. Performance regressions happen. Etc. There should be a more compelling reason to upgrade than the version number changed, given the additional risk you take. Unfortunately, that's often not the case.
