I like multiple figure bases because,
1.) Movement is faster, don't have to fiddle with units 'one figure at a time.'
2.) Easier to settle issues of what goes where, and who's allowed to shoot.
3.) I think that they look better. Sort of little dioramas moving around the table.
OK, there are lots of rules sets out there that handle multi-figure bases.
DBM/DBR/DBA handles it by having only two combat results for defeat. A stand 'backs up' or a stand 'disappears.'
Black powder, I've only played it once, seems to have single figure bases, but the unit frontage is what's important. So you could use multi-figure bases. It tracks casualties by putting markers on whole units.
Death in the Dark Continent, my new favorite colonial rules, has units of multi-figure bases. Most of the time casualties are represented by disorder markers. These affect shooting, morale, the ability to charge, etc. Sometimes, under exceptional circumstances, a target unit will suffer the addition of disorder markers AND the loss of a stand.
I think that DitDC works pretty well.
Regards,
MW