I don't see the problem with Merc/Commandos. Ive played a Mercenary since day 1, it was my first 50, it is still my main. I do fine as a dps merc, albeit I am arsenal specced where as most people prefer pyrotech.
If you can get a healer, or a tank to keep you alive you can kill enemy tanks/healers and dpsers very very quickly as an arsenal merc due to the fact that you reduce armor, deal tech dmg and can hit 2-3 attacks before they even know what hit them.
I have also played a pyrotech and I have to say I enjoy it but not as much as my arsenal. And to the lack of U-T-I-L-I-T-Y as stated previously.
You can make combustible gas cylinders slow the enemy for as long as it is applied which means mercs get a permaslow essentially not sure how that isn't considered utility. Let alone they do massive AoE dmg in all specs besides healing.
Be honest, do you PvP in a ranked setting?
Sure, if you have a healer, you can tear things up.. but so can every other class. The difference is, other classes can tear things up without
a healer. Plus, a rage spec'd Marauder or Jugg will unleash quicker, larger damage than any Merc/Commando could.
A "perma"slow isn't utility. Every other class has a form of slow with relatively low costs, cooldowns and/or in a passive form from talents. Also, massive AoE damage doesn't matter, because anyone who's competent will move out of your AoE as soon as they see the circle on the ground. Again, your class has nothing unique compared to others, and other classes do your job better. Rage juggs/marauders do massive aoe also, and it's instant, + it can occur rather frequently. There's nothing that redeems mercs/commandos at the moment.