Yeah, seems really weird to have team tips for a role that can only be filled by such niche heroes. Maybe this tip only shows up on Defense maps?

This is false. Purchasing items via the Market Board from a retainer stationed in a city that you are not currently standing in will incur an additional fee. This can easily be verified by walking to any Market Board and comparing the purchase dialog that appears when purchasing from a local retainer vs. a retainer stationed abroad; the latter will specify additional cost. As the fee is a percentage of the sale price, teleporting to the retainer's city can realize significant cost savings when purchasing expensive items.

Another benefit to having only one character is a lower monthly subscription fee, but this must be explicitly chosen at your Mog Station.

In the OP's case where leveling primarily via Leves, melding a few cheap materia specifically to increase the Gathering stat may be a particularly efficient and cost effective way to further speed up the leveling process as a number of leves are made significantly easier and more rewarding (XP-wise) as a function of your Gathering stat. Two chief factors will determine whether this is worth their time: easy access to a crafter who can meld for them (i.e. free from your FC) and the cost of obtaining and melding said materia vs. the cost of upgrading your gear approx. every 5 levels.