The JRiver Media Center will never really be "finished", it's an ongoing evolution.
When JRiver opens a beta version to the general public, that means that version is stable (99% free of crashes) but additions (and deletions) of features and refinements are still continuing. At some point, JRiver feels they've tinkered enuff with that version and it's time for a break and "release" it. After the break it's time to continue the evolution and the next version starts and eventually becomes public beta again.
So, when MC beta is offerd for sale, you can figure that it's pretty stable at that point and that there shouldn't be any major changes to the app that would cause any instabillities, (there have been exceptions of course, but these have always been quickly fixed). I think you'll find that the public beta that's available, is, in addition to all the new features, just as stable as the previous version (in this case 11.1).
Try it out, enjoy.