Twilight Garrison's exotic perk provides a mid-air dodge Graviton Forfeit's exotic perk unlocks shadestep (A perk exclusive to the Nightstalker) for free. I think the question you should be asking is: Why is shadestep exclusive to Nightstalkers? The answer is simple. The other classes do not have anymore room for those perks. Make room? Would that be fair to the other classes? No.