Updating patch levels _has_ broken my code, several times, over the past ~18 months. Sometimes it's acknolwedged as a regression and another patch is released fixing whatever went wrong; there has been at least one time that did not happen (I could find it if you really don't believe me).
Yes, bugs happen, in-retrospect-wrong decisions happen. This is life. And this is part of what makes maintaining a handful of Rails apps developed under different Rails versions a pretty labor intensive thing, even with no new features.
Rails maintenance policy is summarized in the OP we are discussing. Rails 4.2.0 is now in beta. Once it is released, support policy is no more security fixes for 4.0. Rails 4.0.x was the latest Rails release until April 8 2014 when Rails 4.1.0 was released. So if you developed an app against the latest Rails minor version in March 2014, within 6 months your app is on a version that doesn't even get security updates, and needs to be upgraded. Yes, it's probably not that hard to upgrade it in this case.
And, yes, when you have a handful or more of legacy apps, and upgrades happen every ~6 months, you spend a non-trivial amount of time on the treadmill. This is my only point. I am not a Rails hater. I use Rails daily. I know of nothing better. There are all sorts of reasons that lead to maintaning Rails apps being such a time-suck, and none of them are that Rails devs are mean or moronic. But it is astonishing to me that Rails inspires a level of 'fanboy'itude that has people insisting that in fact it's not hard at all to maintain legacy Rails apps. Have you done it? With more than one app at a time? With more than 5 apps at a time?