The release of the Blood of Luclin expansion also added a new persistent mercenary buffs. There has, as of yet, been no developer explanation in public regarding these buffs, so I'm quite grateful to Zarinah for pulling the below information and the informoation on overseers from the limited-access beta forums before those forums were removed at the end of beta.
Mercenaries that have been unlocked to hire anywhere will now grant a persistent "Mercenary Battalion" buff even when not summoned. Every mercenary that you have unlocked to hire anywhere may contribute to Mercenary Battalion, with higher tier and higher rank mercenaries contributing more. While each type of mercenary will grant a different buff, all mercenaries of that era contribute to the effect of all other mercenaries of that era.
How do I make my mercenary contribute? Simply summon a mercenary that you have unlocked to Hire Anywhere which is also at least rank 5, and its contribution will be added to the total. Contribution is increased at rank 5, rank 10 rank 20, and eventually at rank 25 when that is unlocked. While you have a mercenary summoned, you can right click on it to check how much it has contributed to the total.
How high can contribution go? Contribution can be increased up to 100% and will not further increase the effects once 100% has been reached.
All currently available mercenaries contribute to the Mercenary Battalion: Shattered Lands effect. Future mercenaries will contribute to new or more powerful versions of Mercenary Battalion.
Do all mercenaries grant the same effect? No, each mercenary grants a specific buff based off of its type. While the buff differentiation is not in the current beta build, the different types are listed below. The buff type should correspond to the mercenary type. Further, more powerful/rare mercenaries provide a more powerful version of the effect.
Word is that if your hire anywhere merc is not granting a buff, the first thing you should try is to hire a different merc, then dismiss them, then summon the merc that is being stubborn about the buff. If that does not work, you may need to /bug that specific merc so that it can be looked at.