There's an interesting natural dichotomy to the whole process. You'll have 'optimized' (sometimes somewhat inaccurate, often with graphics 'enhancements') emulators, but over a period of time a high-end arcade hardware becomes more and more playable in MAME on the new hardware of the era and people just naturally drift to that as the all-in-one solution.
Not that it entirely matters-- it's the accurate documentation that MAME provides that allows for these less accurate emulators to exist in many cases anyway, and MAME is all about documentation as compared to playability in the first place.
That still makes it rather interesting to watch the wave of progress-- into MAME, passing into optimized/high level emulators, then back into MAME.