For shits and giggles, assuming all the software list work is in code shared by MAME, you should try as an experiment editing an arcade driver and loading ROMs via software list.
For most systems it doesn't make sense.. the XML is a lot uglier to use / work with than the built in rom macros, and when you're loading a lot of roms for a game you want clarity, not the coding equivalent of a cave shooter.
However I've already stated that I plan on adding a Megatech option using the Software lists, because the software lists give you multi cartslot support, which means people will be able to specify exactly what configuration of 10 games they want on the MegaTech if they use that option.
There are places where the SLs make sense in MAME, there are places where they don't. As a secondary option for some systems they could potentially be quite useful tho :-) As a primary option, the thought of the XML makes me cringe.
I'd like to see MAME/MESS fall back to the primary rom paths if software isn't found in the system folders tho, that will make the megatech stuff easier, because people aren't going to want to have to duplicate their roms in a 'megatech' folder. Maybe this was already done, I haven't checked yet.