If you already have a system like that, sure use it. However, I don't think most people have systems like that just taking up space.
It's not a matter of "just use it" It just really isn't necessary for MAME only. You could get a computer that will run almost everything for a few hundred bucks. If you really want to add those 10-20 games, then sure, build that system. However, that money can go a long way into finishing your cab.
That's my .02 anyway.
PS - this is all void if you use other emulators or use your cab for more computer intensive uses... in which case I say go for it.