Original big reason on why the Visitor was implemented wasn't to enable markets, but because large villages were infeasible under the w8 new key and locks system:
1) You need to give every villager a slave key so they can exit the village. (In Legacy, you could build a one-way teleporting road sign that takes people outside.)
2) While you can't use a slave key to open a gate from the outside, this means that every villager is a potential "single-point-of-failure" whether though A) planned infiltration B) being fooled by
some kind of trick like this or C) plain incompetence/laziness on closing the gate behind you.
Source