Hunters make their own armor, generally by integrating bits of Titan and Warlock armor together along with whatever they feel they need. Warlock armor is focused on freedom of movement and assisting their mental capabilities. Their abilities are dependent on them maintaining Flow State, a real psychological/physiological phenomenon (google it, pretty awesome). Because of the need of freedom of movement, it actually relies more on the technological side of the armor than the other two, so heavy armor plates aren't needed.

That is pretty neat. although all that flow probably has a negative on defensive capabilities.

Not really. It is complete focus in the task at hand. If that task is combat, should be fine. Their defensive lacking comes from the need for mobility. Those armor plates they are missing do make a difference.