Tavern lets em be creative and generally socialise more effectively, the whole cultural thing, more likely to see/hear something they like in the form of dance, poetry or song/music. The major drawback at least in 42.06 is that you cant actually assign anyone to the roles in the tavern, because they all just love to serve drinks and a lot of your non-dwarves are fully capable of drinking themselves to death.
Couple of obvious points not really mentioned.
The main draw obviously is that you are gonna get some citizen aplications, so heres why you actually want non-dwarves. (and by extension the tavern to attract them)
1. Body Size. Human mercenaries are awesome, especially now that you can use 'd'etails on your forge job to make them mastercraft steel rather than rely on the crap the human caravan brings. They are awesome because they are big, it still plays a fair role in combat, and it lets em use some exotic weaponry. Its also a great way to import skills. I have a squad of master lasher dwarves, originally trained by a long-dead human Lasher who served as a merc for a few years before being devoured by a FB.
2. Cave Adaptation. Humans dont cave adapt, which is why you dont just DAS/reject all the human bards (or I dont). Yes hes an annoying talentless poet, but hes also a menial hauler who will never cave adapt.
I love visitors, I take everyone I can get since I want em all to socialise and have a good time and apply for citizenship. Thats where you get to be a bit more picky and choose who you actually keep. I then often even go to the extent of building a surface fort above for my human residents, they take over surface farming, woodcutting, most of the basic animal stuff, fishing, battlefield clearance etc etc.
With that said I hate elven archer mercs, they are good on paper but you have to make them a dedicated bow squad or you get ammo nightmares (archers with bolts, xbowers with arrows, all sorts of nonsense), and since mercs cant be squad leader you then actually have to assign one of your faithful dwarves to lead the dirty hippies, and he either takes melee or must also bow...bleurgh.
Goblins....goblins seem cursed. I like the idea of getting a goblin citizen, but any that visit my fort tend to end up beaten to death in the tavern at some point, usually by a human or xenophobic dwarf.