It is entirely an incentive to do otherwise unpopular content from the game. It's not unique to retainers either, Eureka has unique furniture, old savages have unique furniture. Like you I'm annoyed the system works this way, but if it didn't then who's to say that part of content wouldn't die? A lot of people I know don't bother with ventures and just use retainers for storage, but having items locked behind leveling it encourages players to use them to their full extent and rewards them for doing so.