From the Rails core mailing list:
But point releases (e.g. 3.1.x) don't add features or change too much code, they just try to have bugfixes. Bugs are fixed by adding a failing test and making it pass, while ensuring the rest of the test suite passes too. This means each point release has less bugs than the previous one. Upgrading to the newest bugfix release is quick, safe, and should be done as often as possible.