I will remind you that Titans are also able to move extremely fast with good mobility. If they were meant to be tanks then surely a mobility nerf is due then to consilidate them in that role.
The role of classes in Destiny is not just simply Tank/Rogue/Mage from most games but instead seems to follow a different pattern instead.
You have the guys who focus on abilities - Warlock
You have the guys who focus on mobility and precision - Hunters
You have the guys focusing on raw gun play and heavy weapons - Titans
Also, Warlocks only have higher damage resistance with a Radiant skin sunsinger, which requires sacrificing self resurrect.
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They nerfed the titans agility too. Heavy weapons and gunplay are not relevant in pvp matches. Heavy weapons start with no ammo. All guns/weapons can be equipped amongst other classes as well.