I remember some posts awhile back saying they would look into increasing the rate mercenaries earn AA experience, and the effectiveness of mercenaries (I think specifically tanks).
Would also help attrition to putting mercs in the game around OOW when the game is largely instanced and attrition is rampant.
Merc AAXP was indeed increased (several patches ago) & the Level 110 Tank Mercs were given some revisions with those scaled back to 100 Tank Mercs. Tank Mercs below 100 have not had any changes that I am aware of.