Merc/Commando are completely viable for rated warzones. I run a merc healer in the top pvp guild(s) on my server. I can regularly top 600k heals in heal spec and 600k damage for both arsenal and pyro WHILE in heal gear. You're crazy if you think merc/commando heals are not useful in a warzone. Maybe you have some other heavy armored single target healer that I don't know about.
Merc/commando 1vs1 could use some improvement. That being said heal spec using combustible gas is your best bet for 1vs1 though I have not tested arsenal in 1vs1 since the last patch. I feel like as heal spec,with practice, the only classes/specs that will give me trouble are annihilation marauders and heal spec sorcs/ops.
Heat management could use improvement as well.