Q6: Are there items only obtainable through the retainer venture system?

A6: Yes, there are!

. . . . .

So, not sure how the venture currency will work and whether we can send them all at the same time on ventures but, yeah..... it will possibly give a small advantage to people who can, or wish to pay for whatever the max number for retainers is.

Initially, in 1.0 they were planned to be available for $$$ as an option to have more storage space. Not giving other advantages to players other than allowing them to hoard stuff and go OCD with collecting every pebble they picked up on their adventures. In ARR, by introducing items that our retainers can get for us while we're afk it's fair to say some players might have a slight advantage over others in game (more items to sell, more ventures-only mats to craft etc.) Gotta wait and see I guess.... Though, Yoshi himself said that there will be an advantage to having more retainers (in that interview translated from JP the other day, to be taken with a grain of salt since it wasn't an official translation).