The way I see it. CoD is O.P. when it comes to using a knife & halo has the hit or assassinate from behind.
During the beta I had to stab a titan in the back of the head 3 times & a warlock 2 times to kill them. I'm a huge fan of being rewarded when I get the drop on some one. It feels like I'm only rewarded for shooting instead of being sneaky.
*edit for clarification:
performing a melee strike, while flanking your target, should be treated like a skill shot (all classes).
time wise: 1hit vs. 2-3 hits
(outside of beta cap & hunters sub class)
destiny is a game trying to not define a class for specific rolls. people can play all classes as: snipers, shot gunners, grenaders, melee juggernauts & sneaky assassins