Weapon Mastery was more of a bridge to Power Mechanics if I recall correctly. It was instituted to allow all players a way to do the competitive damage to the heavy hitters, i.e. Light, Celestial and Rage. It was introduced, as you mentioned, to allow players to also play similar role-play to Deathstroke and other multi-weapon users, but its function was something in the meantime while they tried to change other power sets to provide heavy damage. When I say they should have made changes prior to tier 6, I mean that we really didn't need everything we've had in between GU 37 and GU 73 in terms of power revamps. All we really needed was for them to do a combination of update 36 (with Weapon Mastery) and update 73.