Why is shield swipe so bad?

[reality_check]

Also it does excellent damage for the cost when you have a huge threat lead.

Why do people seem to think shield swipe is excellent damage? Average Paladin GCD ability does 206 potency, by using a GCD on a 210 potency you gain 2% damage done for THAT GCD only. Given that the ability procs (conservatively) every 10 seconds, the dps increase by using shield swipe is even lower than 2%, like significantly lower.