As far as I knew, you could only parry magical "auto" attacks, like ice sprite Blizzard casts. I'm also not sure that was confirmed, or if it still works in Heavensward--I tried to get some parries with Keen Flurry against a sprite, and didn't get a single parry out of a 6 minute encounter.
Anyway, stacking parry is bad because you could have determination or crit rate instead. Instead of a chance of 20% mitigation (massive nerf from 2.55), you can have a 100% mitigation by getting rid of the source.