My troll stick (aka my puri wand that I didn't even want) on my mystic has never procced. Ever.
My friend and I did a test on this a few years ago on PWI with the R8r weapons. The proc rate has several different variations, one of them being based on damage taken. We found that the lower the damage, the less the weapon would proc. However as the damage increased, the more the weapon would begin to proc. We ended up coming to the conclusion that it's about 7-9% to proc with average damage. However, this was all just a test, and I'm not for sure whether or not it's the same here.
What a load of crap, if only you knew the amount of times purify spell has procced when I've used whirlwind..