3rd Retainer by Default, please?

[Aerowaffle]

I completely disagree with you, there is not a gameplay advantage, there is a convenience factor. If I were unwilling to pay for additional retainers, it would not alter the gameplay at all, I would simply have to alter my priorities as to what materials and gear I keep on hand, and be a bit more organized. Having the additional retainers allows me to be a bit more lazy about my inventory space and flexible since I have the space. That is neither a competitive advantage, nor different gameplay.

Convenience factors are advantages. Your mount is a convenience factor, you can reach any place on foot as you would with a mount, but it's considered an advantage to use a mount over walking because you get to locations quicker.

Just as having more inventory would allow you to be less picky on what you throw away (costing potential gil in the process). Having additional retainers also gives more than just inventory space, it gives a possible advantage on the market by being able to sell more things. More retainers also means more possible items retrieved by ventures.

If the goal is to solve an inventory problem, additional retainers gives more advantages than just larger inventory. An inventory problem should be solved with some sort of new storage chest, or a few more tabs added to retainers in general.