I'm a creature of habit. Since pre-BC, I've always carried a stack of each type of poison around in my bags right next to my other necessary knickknacks (hearthstone, lockpicking tools, skinning knife, bandages, food, Iron Flask, Worn Troll Dice, etc.). Some of those items have since become obsolete, and I've eagerly chucked them into the bank since I'm also a pack rat who won't throw anything away (e.g. I still have my lockpicking tools in the bank).
And yet, I still have a stack of Crippling Poison in my bags. I play as Mutilate both in PvE and PvP. In PvE, you never, ever, ever ever ever have need for things to be slower because you can either: a.) stun them, or b.) kill them before your Cheap Shot wears off. In PvP, there's Deadly Brew, which allows you to apply Crippling without ever putting it on your weapons. To be honest, I'm not sure why I keep it in my bags, although I'm certainly in no need for extra storage, even with a whole bag full of extra gear and another full of quest items that I don't want to constantly run to my bank to retrieve on top of the aforementioned bag full of necessary knickknacks.
Which brings me to the question: when was the last time you actually applied Crippling Poison to your weapons?
For me, it was probably in BC when I still PvP'd as Shadowstep. Since the pre-WotLK patch that brought us the glorious Mutilate renaissance, there's simply been no need to use it. I'm sure that somebody still uses it, but I can't imagine who or why.