damage wise shield swipe does more damage over time than your 1/2/3 combo if you could use both continuously:

Fast blade= 150 pot, Savage= 200 pot, and Rage=260 pot, Average pot= (150+200+260= 610)/3= 203.33 damage over time Vs 210 potency of Shield swipe It also doesn't interrupt the cool down and has low Tp cost. The only time i think this isn't true is if you have a warrior iwth storm's eye up on the boss because then you combo gets +10% damage. Also seeing as how threat is very manageable if people are using their abilities effectively; you should most likely be able to cast shield swipe every time its up and still be top threat by a lot.