Convenience, yes. Advantage, no.
There is no gameplay difference or change in game play due to having an additional retainer, it does not affect combat at all, nor does it impact on my ability to gather or craft. It does make inventory management easier for me because I do not discipline my use of inventory space, but that is not a gameplay advantage, it is a concenience.
A retainer cannot make anything at all without the player doing something. If the retainer was going out on it's own and making 100K gil without any intervention by the player at all, it might be an advantage, but since the player has to actually do something to make that happen, it's not an advantage, it's simply content to play.
How is that retainer a gameplay advantage? Does it confer any special buffs or capabilities on the player? Does it make them faster on the ground, or able to teleport to any point on the map? Does it give them the ability to ignore attacks? Does it increase their HP? There is no gameplay advantage there.
You could argue that it makes it easier for a player to earn gil on the market boards, but at the end of the day the player still has to work with the game to obtain the gil, it's not automatic. So it's not an advantage, it's a convenience for the player. If you're saying that having the extra gil is an advantage, then every player who was ever fortunate to obtain any random drop that could be sold for millions of gil has an advantage, which is ridiculous.
I know many players who leveled all their crafts to 50 within months of ARR's launch. These players have earned millions upon millions of gil through the market board - and were doing so before the additional retainers came along. What advantage has their extra gil afforded them? A large house and the ability to purchase some decent overmelded gear that was quickly made obsolete by the next patch. of course they never really put the gear to good use since they spent their time crafting and playing the market boards instead of farming coils in a static.
There is no advantage here, it's a fallacy to suggest that the availability of additional paid retainers offers a gameplay advantage, it does not.