Given that a majority of users (and devs) don't want the combined build, and that it would greatly increase bandwidth costs for MW and our other mirrors, it would be dumb to do it.
If the size is your concern, than main official build should contain no bootlegs, gambling, and maybe even no neo-geo games, just optional at compile time.
Actually, if it was up to me I'd get rid of almost everything... the artwork, bezels, cheats, screenshots, scanlines & HLSLSLHL effects, and anything else that is not ABSOLUTELY necessary just to run the games as the original PCB would. Anything doing more than that I would put in some kind of add-ons, just OPTIONAL to this core MAME.
People working on, say... artwork, will not then be able to screw up already working game drivers. I'd say these things should really be separated from the main core build much more than MESS. MESS should be treated like NEO-GEO.