We just recommend not to use SMF 2.1 on production sites because as it's an under-development version, you may encounter errors and bugs. Although some people decide to run it, it depends on your skills in general. There are some people that maintain their production sites up to the latest GitHub build, some others up to the latest release (RC2 in our case)... It just depends on what do you want and what how much are you willing to risk based on your skills.
Yeah, I think I know what I'm doing (and have seen and reported my share of bugs), although I'm not strong and PHP and always short on time.

My small user group was involved in the decision and preferred being beta testers. (Also, "production" in my case just means "a bunch of friends needed a private forum for a hobby", no SLAs or anything, just unpaid volunteer work.)
If you have 2 sites and one of them is in production, I would recommend sticking with RC2 in the production one and keep the other one updated with latest GitHub build as a playground.
The point of the staging or "beta" forum is to be exactly the same as the production site, only with less users and no (precious) content, to test any small to major change before touching the production forum.
I'll try to update my third, previously unmentioned, throw-away "alpha" forum to HEAD once I've figured out a way to manage my changes (I will probably just put the forum dirs into different git branches later this month, to be able to cherry-pick file changes across them, and because I have no better idea right now).