The best part of this buff, besides the proc, is the defensive proc that comes with it. Every time the pet does an offensive proc (which is very frequently), it also gets the defensive proc from it (buff says Prism Skin) which absorbs 2 melee attacks. When you are fighting mobs that can quad for 2-3k, absorbing 2 of thos blows could mean life or death. Couple that with our pet 'rune' on 79 earth pet, you got you a lvl 80+ tank. I have been in a couple groups now in FoS w/ 80+ tanks (Pallies, Sk's, Warr's) that b/c of these buffs, my pet was a more efficient tank than actual PC's! I can now solo (again, w/ merc of course) mob's in FoS that would otherwise eat my pet alive, as well as all the named in OGV. I was able to take on an ancient bonegrinder, and if my merc had not ran out of mana, it would have been cake (got him to 75% solo!... then very abruptly died a horrible death).
Note: My pets are always fully buffed, Unity, Cleric buffs, Armor, etc.