
Visitor gates are designed to not allow anyone in combat to enter or exit a visitor gate.
However, if you build a visitor gate built bound to a cave entrance, you can violate this rule, and enter while you are in combat, and others who are in combat with you can also enter.
However, you get all the benefits of the visitor buff and they get all the penalties.
How this plays out in practice:
Attacker aggros some people or ganks some newb sprucecap, then waits for rangers, aggros them, then runs into cave. If rangers follow, they are immediately hit with visitor debuff. Unable to exit. Unable to vandalize mine walls. Unable to log out because locked in aggro. Attacker simply stands behind objects that can't be lifted because of theft. Then they can hold you hostage indefinitely, until they decide to come by with their gank squad. You can't aggro them, they can aggro you, so even if you sent people to help, they will just enable murderous rage in complete safety and take you out one at a time.