I can't speak for JRiver, because I don't work for them, but I know how it works because I've been here a long while. You have, too, csimon.
Each major version of MC is developed actively for about a year. This year is divided into several "sections". Early in the development cycle, they are "free" to make major changes and add new features, which often breaks things. Later on, they slow down on adding new features and try to focus on "cleaning up their mess" (finishing the stuff they added and fixing the stuff they broke while making changes). Near the end of the development cycle, they add almost nothing, and work almost exclusively on polishing and bug-quashing (sometimes releasing minor features, or things that had been in development but unreleased for a long while). Then, at the very, very end (as they start on the new version), they usually put out a handful of new builds that fix up a few last issues discovered near the end.
We're in this last phase now. Then, after a few more builds, they stop development on the old version and move the team entirely to the new version. Does this make the software absolutely perfect in every conceivable way at the end? Does that mean they fix every single thing reported by every single customer? No, of course not. Some things are discovered after they're done, some things are minor enough or only impact a subset of users (or are just too difficult to fix without causing lots of new issues), and some things they just decide aren't worth addressing for their own reasons. But, it does make it a stable platform that can be used without changing over the next few months while they break things in the new version and try to make us new, cool stuff to use.
All software made by humans has bugs. The copy of Adobe Photoshop CS5.5 I have has a list of known issues that can fill a short book. The copies of Polycom RealPresence Resource Manager, Digital Media Application, and RMX that I have, which I paid a quarter-million dollars for, have a
literal book of bugs and known issues (and others that aren't in the books). If I stop paying the maintenance contract price every year, I stop getting upgrades (which fix some bugs and make new ones). Even hardware "code" has bugs. Every Intel CPU revision has a list of Errata they publish (and update as they find new things) in the
Processor Specification Update. Some of these are fixed in microcode updates, some have published workarounds (which often amount to "don't do that"), and some
require them to turn off features and fix it in the next go-round.
Because it is software made by humans which are imperfect, running on software built by humans which are imperfect, running on hardware built by humans which are imperfect. There's
no such thing as perfection in this world.