I'm not sure what the deal is with Crawling Infestation.
My friend got it on one of our playthroughs, and we tested it out. I had to use Drain Willpower and Soulsap on an enemy so that it wouldn't succeed at a Body Building saving throw, then my friend used Crawling Infestation.
I froze the enemy, and two turns went by. Then my friend used Divine Light to lower the enemy's Body Building again so that he wouldn't saving throw out of Crawling Infestation. I stunned the enemy for two more turns, and after those two turns, when the enemy became unstunned, the spiders came out of him. During that whole time, it had shown under the enemy's status effects: "Crawling Infestation: 3 Turns". The turn count never decreased, and it actually took like five turns instead of three for it to take effect.
We then reloaded a save and he chose another skill to get instead of Crawling Infestation.
Seriously, most useless skill in the game, by far. First of all, it takes at least three turns and has a Body Building saving throw, but the most turns you can lower Body Building for is 2, so unless you do:
(1) Drain Willpower
(2) Soulsap
(3) Crawling Infestation
(4) Freeze / Stun
(5) Pass
(6) Pass
(7) Divine Light
(8) Freeze / Stun
(9) Pass
(10) Pass
...the enemy will just saving throw out of it anyway. It has such a low chance to proc, even with high Dexterity, that the only way you can guarantee success with a relatively high chance is to waste three debuff skills, two CC skills, one master Scoundrel skill, and like four turns of waiting, all on a single enemy. Keep in mind that this skill basically has a 0% chance to succeed unless you have a Witchcraft mage with Drain Willpower and Soulsap, but if you have a Witchcraft mage, just do Drain Willpower and Rapture instead of wasting nine more turns of useful skills just to take down one enemy.
I dunno WTF they were thinking, this skill is the epitome of uselessness.