Parry is a viable PvE combat skill that is calculated as follows:

Base chance to Parry = 5% + (0.5% x Parry spec) + 3% per level of Mastery of Parry.

It is further modified by your dexterity, such that kobolds parry considerably more often than trolls. It only works in the forward arc, its is reduced by 50% if your opponent is using a 2 handed weapon, and is drastically reduced in RvR with the current group code penalties.

More opponents in Shrouded Isles use styles and 2 handed weapons so the effectiveness in PvE is being diminished as well. Its use in RvR is poor.