On topic - the 68XXX.library download saves a little RAM (still work with MMULib, not for standalone use).
They also appear to address issues with superkickstart, ACA boards, and more support for Phase 5 boards (which don't all autoconfigure the same if at all).
The actual functional part of it is quite small, most of it is source and documentation.
It's optional, not required, but does save a little RAM and tries to be more compatible.
All I did was read the write up on aminet. While I do not generally use MMULib, it is there for people if they want it, and it does help with stability and greater compatibilty. At the expense of using RAM but you can''t have everything.
The reason for people using it is because they want to get the best out of a 68040 or (especially) an 68060. Nothing wrong with it. It can be used with other processors.