Well, if you don't put any tables in your tavern and just leave it as a performance hall, you might get some mileage out of it. As for why you shouldn't just leave extra room for a dance floor to the side of the tables... aka a complete tavern? Not really one, no. Taverns can completely replace dining rooms. If you want to decentralize things, you can have several, smaller dining rooms spread out throughout your fort, then just one big tavern near the entrance you can. Or you can make a couple small taverns as well, but make them citizen-only and leave the big one for everyone.
Personally, I see no point to separating them. I will make smaller citizen-only taverns spread out throughout my fort, though. Near the mines is one, near the caverns is one, near my forges is one... They are all small and I really wish I could move one of the two-thousand entertainers that won't to perform in my main tavern to the satellites, but that bug hasn't been fixed yet. I really hope Toady gets around to fixing that "can't ever assign residents/naturalized citizens to positions" bug. The only thing we can do to them is put them in the military, but never as a captain. Hilariously, long-term resident performers will even stop performing when you grant them citizenship- and you can never reassign them to it.