It would be nice if all people were so logically minded, and could think rationally about pros and cons of running a battle-hardened piece of code from last year, being guided in their decisions only by the unshakable faith in the values of critical thought and Scientific Enlightenment as declined by our Engineer-in-Chief.
The truth is, we all know that the average geek, giving a choice between $software version 2012_11_20 and 2014_12_15, would almost always pick the latter. Arguments that "the 2012_11* codeline is a workhorse and runs better with our hardware!" would simply not cut it; newer software will have more bugs fixed, right? It's only natural. Of course "nobody would run 2015_02_13, it's too bleeding edge", but a couple of months should be fine, surely? ... This sort of thought is not even conscious in most seasoned geeks, but it's inevitably there. Anything older than a few months would quickly lose all significance.
A date will irrevocably reduce a piece of software to a moment in time, an idea that a release is just a timestamp on a single line going from A to B; a date-agnostic release number gives software an identity that transcends time, so that it will actually make the choice clearer: you run 2.x because you want some features and not others for your own purposes, regardless of when they were released.
Man is what it is, and despite all their protestations, engineers are just men. If you think this sort of process does not happen in our minds, or that it doesn't matter in the end, then you should try replacing a release manager with a small timestamp-checking shell script.