Powertech has taunt makes some fights a lot easier i.e Firebrand/Stormcaller I wouldn't swap for another Mara.
Assassin tanks can stealth res in parts of the fights where 1 tank is required or a dps can pick up the boss for 30s (powertech)
The dps taunt isn't required. The tank on stormcaller can taunt and take the debuff. Firebrand doesn't do any damage before he casts incinerate armor.