First guess as plausible reasons (and variability) of the problem(s). Read this and if you can try the SMC reset.
I have done an SMC reset a number of times but for the first time, actually started to read some of the detail of what an SMC reset actually is!! http://support.apple.com/kb/HT3964
You could interpret, after reading this as a "catch all" of "bugs" or things going on, messing with the internal abilities of our Apple Macs to manage itself properly. Apple promote the SMC as a last resort really but many of us found - for a while at least, it can provide a serious increase in battery life. Some find the SMC reset a more permanent fix whilst others see it's a real-short term improvement.
So, on this basis, my plausibility theory is that something going on, with certian machines, with certain enviroments that's causing the system to screw itself up in a way Apple say we need the SMC reset at all. i.e. they don't know enough to catch the cause but here's a solution that minimizes the symptoms. Make sense? It'll explain why this is soooo hard to diagnose!
i.e. for some, not all, going from Snow Leopard *OR* Lion to Mountain Lion, or running some app, or some sequence throws ours sytems into one of these "faults". Creating this same condition *would* therefore be fair to conclude, it's throwing the same "faults" to require the same "SMC reset fix".
My latest reset of SMC has helped quite a bit, although the last 3 times I did this, my battery life crept down quite quickly after a few days. If this conclusion is correct, if I "stay" in my newly segrated Mountain Lion partition, after an SMC reset I probably won't see it get worse. However, if I go to my existing ML partition, I suspect something I do, is causing the problems creating the symptoms in the above link.
Anyone else up for trying the SMC reset a few times over the weekend?