This surprises me as I note that 12.0.533 was released on 8/29/08 and 12.0.532 was released on 8/25 and the prior release on 8/18. Where those "show stoppers?"
Should we not expect any non "show stoppers" after 12.0.533?
JRiver has tended to go a few builds past where Jim says they're done. Usually because when Jim says "we're done" then people come out of the woodwork with all kinds of weird stuff that never got brought up before. (Even though there is always a thread beforehand saying something like "we think we're almost done... so speak now or hold your peace." But, you know how people are. Never motivated until they're motivated, so they usually take pity and throw us 3-4 new "almost regular" builds before focusing on the new version.
If you look at the change logs for those builds you mentioned, though, those changes were all either bug fixes or small tweaks. All new true "feature development" effort has been on MC13. If history is a guide, they'll keep bug-fixing little things here or there on MC12, and builds will become fewer and further between. Don't worry though... MC13 will be along before too long.