Retainers at the moment are limited to only Market Wards, which is also limited by a retainer occupant cap. It is literally impossible to constantly expand the "dispatched cap" [dev1114] on a given ward infinitely. Allow players to place retainers outside in the open world would allow players to create a pseudo-market wards (flea market) where ever they choose to with their retainers.

Alternatively, limit retainers to certain areas in the open world, such as hamlets, or merchant streets in the town. This way it at least seems like the world has a bit more life to it with user's interactions changing how the world looks. Rather than a depressing small room filled with all the retainers.

I'm only assuming this, but the game would eventually reach a point where bazaars would become similar to FFXI's. Such that, regardless of retainers, people would actually end up using their bazaar to sell useful items for other players, and station their characters at certain high traffic areas.