I started out using balance on my healer merc thinking it'd save mana and such and that that would be needed Then switched to reactive when it looked like the problem was more of the healer not healing fast enough. Maybe on parse (if you can parse it), there is a difference, but I could not tell any difference "from the seat of my pants" using balance or reactive. When I did HA nameds at 85 to 92ish or so, the healer would go OOM and I'd wipe, even with using my shaman to support heal. But that was a DPS problem or a mob tuning problem. Now, I never see my merc healer low on mana and I always leave the healer on reactive. They regen very fast. I pull and tank with my warrior, so the mercs always has a second or two to meditate and seems that's all they need.
At 85 (I started back up with all heroic characters) and a fresh zero AA merc, I did get my healer merc low on mana as well as the caster. At 105 all AA's maxed, I don't think I even see them drop below 80% for experience grind. I am sure a group with a dedicated tank and a fast bard puller running around grabbing more mobs while the group is still fighting can get the mercs low on mana, but it seems next to impossible for me to. And I am not slow on pulls, I get all the DH HA's done in 25 minutes or less, it's just I can't tank and pull.
Sippin, the majority of the time, the mec healer does great and I hate having to concern myself with healing. If it were only my tank that I had to heal, no problems, but occasionally you get adds, over-agro, what ever and now my enchanter, shaman, wizard or caster mercs or the healer himself or a pet needs a heal. This is where I fail hard at healing. When I use to box a cleric, by the time I noticed anything other than my tank needing a heal, it was already too late. The merc healer actually does great at healing the non-tanks when agro is lost on my tank. And since I blocked HoT and promise line of heals on my warrior, I haven't had any problems with my healer being too slow to heal.
Edited, Jul 1st 2015 10:36am by Boop