I havent had any issues with this. I myself am a proficent Merc Healer and a DPS Operative (working on a Marauder) and have never had any issues with Merc Healers healing properly. One thing we need is a View Specification in the character sheet so that you can check if they are truely healer or not.
merc healers can heal just fine.
mercs who don't use a single heal aren't merc healers.