I have been a Lightroom user since the very first early Beta versions and had decided to upgrade (and actually downloaded) Lightroom 5, only to find out that it won't work with my version of OSX 10.6.8. I have everything running so smoothly and my workflow is so smooth with my existing version 4.4 that I just don't won't to take the chance with a new operating system. I know I will eventually have to make a move but there are no compelling/must have features in Lightroom 5 that I want to go to the trouble. I will continue to use Lightroom 4.4 but will probably now add Capture One Pro 7 and do some testing with it.
Certainly your logic of "if it ain't broke don't fix it", but I think your fears are a little unfounded. Mountain Lion is a year old, is on it's 4th release, and most of the issues with Leopard/snow leopard had to do with printing, and weren't issues when moving on to Lion and Mountain Lion - once you solved them you were fine because the printing pipeline really didn't change with Lion.
There are over 1500 api's and other tools available to developers to tweak more features and performance out of their apps than previous OS's, and LR 5 and other programs are leveraging those.