Ask a Thief Thread.

Web immobilizes monsters, and has a poison effect. The poison effect takes 40 seconds to take it to 1 HP at all levels, however the % chance that the attack actually activates increases as you level it.

Shadow meso is it's own attack. You throw money, and it does damage. Pretty good damage, but not a lot. Used to be useful for KBing pap, but that's not much of an issue nowadays, so it's rarely maxed.

Most people make it sound like they are crap, but those seem like useful skills to have. Like, immobilizing monsters when you're out of stars and throwing money at them, then running like hell. Or using Shadow Web to hold monsters down so you can use Avenger on all of them.