Mercs rule 2v2 because they have the best multi- scratch that, the only good multi. Their multi is powered by the best stat in the game: Support. Even though you can't increase its damage with a skill, its damage can't be lowered other than by a shield. Even though it can't be powered it, its still so much more powerful than the other multis. And not being able to be powered up with a skill is more of an advantage especially with its high base damage, it means you don't have to waste energy and skill points to be able to make it more useful. The Bounty Hunter and Tech Mage multis just can't stand up to Artillery Strike. Its one of the best skills in the game while the BH and TM multis are pretty much the worst. Artillery strike also has 10% defense ignore, making it more powerful.
Mercs also have all the right attributes to dominate 2v2. The Bunker Buster + Artillery Strike is so overpowered with high support. Bunker Buster = 25% crit chance with high support will do alot of damage, Artillery strike will do 40 - 60ish with high support and after that the Aux will do high damage as well. That can result in the end of the battle in some situations. Two support Mercs in a team in 2v2 against 1 Merc and a BH/Mage or 2 BH/Mages is an unbalanced battle. Tech Mages can do okay in 2v2 but they will never be able to do what Mercs can. Bounty hunters are just not made for 2v2, they don't have the right skills and they mostly rely on stats to win battles.
A Techincian + Mult or Reflex + Multi combo just costs far too much energy, and in the end Artillery Strike will still be more powerful. 5 focus Tech Mage is really bad unless the player really, and I mean really, knows what they're doing and is good with strategy. Mercs dominate 2v2, I doubt that'll ever change.
Few things here
1)You're right that a multi can't have it damage lowed, and that sucks. However, as a 5 focus non multi 2v2 merc, I've never had a non crit multi do more than 30 damage, unless it was a crit. The 10 percent defense thing really needs to be removed, or at least add some extra effects to bounty/mage multi's, multi skills are not something that should have a hierarchy.
2) a merc carrying both bunker AND multi will fail 90 percent of the time. They carry to little energy, making them a prime target for a nice atom smasher, they lack enough skills to deal to with most situations, meaning after their energy is gone their pretty much done, and they can't effectively use both as well as someone who either has a crap load of tech or support.
Imo, you can be a support merc, or a tech merc, but never both.