Are the atlas merc passives mandatory when farming mercs?

Hey, so regarding the following effects:

"Merc found in your maps have increased aura effect / have a random shrine buff / have onslaught"

Do these buffs remain with the mercs after you hire them making it mandatory to have these points when farming mercs ?

Otherwise, if they don't stay, what would be the purpose (is it just for the current map) ?