Edited to remove quoted spam post, which had been deleted - Iris.
The beta3 is quite usable as it is, and beta4 is startet upon- on Github at least.
I am developing a new SMF theme for 2.1 beta3/beta4 and see no problem starting to use this version. No need to wait until RC versions - the more that use betas the quicker any issues can be dealt with and the sooner we can get to that stable RC version. I remember using 2.0 betas long before any 2.0 stable versions and there were quite a few changes in the default theme back then, from one beta to another.