Shield Swipe does not silence, it pacifies. This means melee mobs can't use their abilites, but casters can keep right on casting.
I don't like to do this because I find that having the GCD out of your control at any point to be a major flaw. When threat is low and mobs are attacking allies or are about to turn away from you, you can't spare the time to SS. You need to get your FB>SB>RoH combo out right away, and a macro like this could ruin that and wipe the party in the right circumstances.
I used to use SS every time it was up, but now that I'm at the higher levels threat management is becoming more difficult so I have to pick and choose my moments. I use it for the cheap damage output when threat is secure, but that situation is becoming rarer with every level.