I think this depends on how retainers are leveled. I think some people are thinking as you are that retainers will level gathering up to 50 or some equivalent. Another possibility is that a retainer may have a range of lets say 5 levels and as it levels up each level unlocks new abilities.
For example
level 1 - Can be placed in Market Wards (or 2.0 equivalent)
level 2 - Can be placed in Market Wards and undertake shopping expeditions
level 3 - Can be placed in Market Wards, undertake shopping and gathering expeditions
level 4 - Can be placed in Market Wards, undertake shopping and gathering expeditions and house gardening tasks
level 5 - Can be placed in Market Wards, undertake shopping and gathering expeditions, house gardening and chocobo raising tasks
Again all the above is totally made up to illustrate a point. As an awesome but totally unlikely thought, how cool would it be that once your retainer 'capped out' its level you could change it to a pet avatar, gogo retainermon evolution!!! XD