In general, when bumping from a "u" release to a main release, they don't change much if anything. The big releases are supposed to be more stable, so the u release is out for a while and if there are no big bugs they promote it to a main release. Lots of people skip the u releases because of this.
Nope - It's supposed to work like that, but actually it usually goes the other way -
They do a big realease, people report bugs in it, they fix them in a u release, then they start on u releases that actually introduce new changes.
Actually, I find it ironic that they used to do a Beta version, then a realeased candidate, then a major release, but the major release always had some bugs, so they said "Heck with this, MAME is always "beta" and quit doing main releases, but now the U releases are basically the same as the old Beta releases.
BTW, people used to skip the U releases b/c they used to release them officially only in source code and you had to compile your own.