So, my Talented Piggle has these 4 talents: Spell-Proof, Spell-Defying, Balance-It, and Spritely. And, of course, it never casts Spritely, at least in pvp. All of its stats are maxed out, and I see pets cast it much more then mine. I think that boosting up its Will or something makes the pet cast more, but it's stats are maxed, and I don't want to go and try to get a 'Perfect Pet' all over again. So, is there a reason that some pets cast Spritely like 10 times more then others?
I have had spritely cast multiple times in A TURN (cast a AoE DoT -like fire dragon), and each damage tick has a chance to trigger spritely.
I have also gone through dungeons with multiple floors, and have did not have spritely trigger once. (and yes, I wished I could poke him or something. It was kinda like he wasn't even paying attention)
So, I don't really agree that some pets will cast sprite more than others. I think if you looked at it over a long period of time (numbers of damage hits vs. number of spritely casts) you will find they are pretty even. For every major pet-sprite fail, I can think of an example where it really saved me!