When I tried to make a visitors only tavern I also tried to make a vampire detector that would work contrary to at least 95% of all advice, i.e. it was intended to capture/detain/kill vampires only and leave other visitors unharmed and without manual reset intervention for each non vampire (manual handling of vampires, once caught, was deemed OK). Both tasks failed.
What I wanted was a visitors only tavern with automatic screening of everyone leaving the tavern to go to the fortress itself, with potential visitor weres messing up visitors, not valuable fortress members. Another potential use for a visitors only tavern is to use it for Visitite mining, but that would require the horrible task of using ordinary burrows to separate the poison distributors from the rest of the fortress' population.
Visitors, in my view, is the undesired by-product of the process of acquiring scholar and worker (a.k.a. performer) petitioners->citizens to broaden the racial assets available to the fortress. Also, I consider visiting scholars to be valuable, despite their kleptomania. To a very small extent visitors might provide some cultural influx, such as new poems/songs/dances... Some people (but I don't know about dorfs) may find it somewhat tiresome to hear "Raw Hide" for the 21:st time in the row.