They should make the invisibility like in Halo 3 Matchmaking. When you were like almost completely invisible. I got so many Beat Downs and Assassinations.
As for Armor Lock it's good becuase it keeps you alive longer and rejects damage. But yeah, mostly to waste the enemies patience and strike back when they are unawair.
Sprint is what most people use, just basic, for people to like to get assassinations and to places faster. It's mostly used to get tons of kills with the energy sword.
Jet Pack is well for, eh, people who i think just like to soar high and attack from above. Mostly for getting to high places faster.
Hologram is to lure enemies into the battlefield. And to mostly turn the tables on people who like to assassinate.
Drop Shield as i've seen is used for Extra shielding and to increase your health when down. Used mostly in Invasion by Elites and in campaign missions.
Armor Lock is awesome people just get angry when they are used against with it