This way, we have a way for people to test drive the Mercs they want, without having to wait for rotations, in a real server environment rather than alone on a map. We also have the effect of increasing variety and unpredictability as more Mercs are likely to be in play by more people, reducing Merc-spam.
There are several ways. “Rent-a-Merc” could have us pay a small amount to temporarily unlock a Merc, optionally so Credits earned as those Mercs automatically go towards their purchase price, as a fun, intimately personal way to unlock that Merc you wanted.
There could also be a severe usability limit with unpurchased Mercs, such as an inability to earn Credits unless all the Mercs in your squad are already in your possession, and/or the privilege of using a Loadout Card only comes with a purchase.
And finally, the age-old Individual Rotation suggestion. Every player gets their own personalized rotation, optionally with the ability to choose one of the rotated-in Mercs.

