The problem is not so much that the builds are unstable or such, it's that noobs only tend to look so far as the numbers and go "Ohh, 3.6.14, that's what I'm using!", especially, this causes problems when mods use things that are not in the RC but in the final, or when people are trying to play multi together, and, unless the troubleshooter is looking for it specifically, it may be missed, leading to repeated long term problems with tech support because people who don't really know what they're doing have gotten their hands on experimental builds and expected them to be 'release' builds.
It happens far more than you'd imagine.