I use old Macs all the time as mostly "dedicated" mames. There are work arounds when I work with old macs. The first is attempting to run the game(s) I want on OS 9 and an old version of macmame os 9. Everything will run much, much faster on OS 9. Vector games emulate very well on macmame OS9, better than any PC emulator besides AAE; but again, no real front end and limited encoder options. Right now I'm building a Midway mini/cabaret cabinet that runs all the vector hacks for Galaxian, Galaga and the Pacman games. It turns out that Mistfitmame for mac only runs on OSX and the games only turn to the RIGHT vertically. So now I have to unmount the imac G3 400 I installed and flip it upside down so all the games look correct. ACK!
Some games like Neo Geos and Topgunner will only run on Macmame OS X as far as I know. Macmame OS 9 can give that nice "soft" look to the classics and like I said before, OS 9 is KING for emulating vectors IMO.