Damn, poor Mercs, having a threat drop on the gcd. We Guardians are sure lucky to have one that not only costs resource, but continues to deplete our resource bar if we don't cancel it by searching our buffbar for it and hoping to not accidentaly cancel our stim or something.
And weee, Powertechs dont even have a threat drop, isn't that so much better than having one on the gcd?
Bottom line: Different classes differ.
You forgot to mention sins have no threat drop either. (and no combat stealth is NOT a threat drop, you completely exit combat)
Sin, Jugg, PT ... notice a connection?
They're all the classes that have a tank spec.
You don't get a threat drop because you get a taunt instead.
Also my commando threat drop doesn't seem to be on my GCD so I guess it's a bug for mercs.