I think MESS is more or less the MAME of every other system, though. That is, it's very generic and unoptimized. You couldn't play much in the terms of games in it...it might be a bit of a pain to add controls individually to each system for it. It'd be far easier and much better overall to port individual emulators (unless you actually wanted a PET or Vic 20 emulator or something, in which case, maybe it's a good idea!).
MESS's only real quality, as far as playing games is concerned, is for obscure systems. Try finding a better emulator for an Amstrad PCW, for example. You might think it's good for PC but I don't know anyone who ever got it successfully running PC games (and a fair few have tried). To be honest, I don't even know if it could be ported badly!