it's added complexity and in my experience users struggle enough with "just use 0.78 + mame2003", never mind "use 0.78 and mame2003+
I would tend to agree with @barbudreadmon. Even mame2003 is starting to "drift" with changes (albeit small but still no longer the static set it used to be--a few additions here, a rom set rename there). You might as well go with mame2003+ for the most part.
If you can get the ROM management concept down for those minor changes in mame2003, that is if those games interest you, then mame2003+ isn't that much more work.
I would have rather seen mame2003 hold a hard line on its set and let mame2003+ be the dynamic set with additions, deletions and corrections.
I think mame2003 is much simpler to understand and configure (mainly the interface) but mame2003+ is more progressive. They do have 3 folks doing quite a bit of work currently with some worthy additions.
As barbudreadmon pointed out a majority are the original rom set but some updates improve accuracy (like Bubble Bobble) and some additions are recent emulation code, hence more accurate. When you think of the target platform of low spec hardware it's a worthy core. I have a great base game set on the Pi 3 that I'm happy with, and the Pi 4 provides some nice 'oomph' for other cores.