So the spider isn't exactly the best of creatures, but they do have a surprisingly strong little set of spells, the most important ones earned at early levels. Just out of curiosity I did a test, one on one against a giant. All creatures were level 10 and I tested spider, bile demon, orc and dragon.
I did it 20 times with each, and I have to say the spider was the most consistent one. It beat the giant a couple of times but for the most part put it down to 1 to 3 pips of health before it died. The other creatures lost for the most part too but it varied greatly between tests. The dragons failed miserably everytime though.
I know that's a very specific situation that doesn't say much, but really, the freeze and slow spells really are potent. Most enemies (including the avatar) don't have rebound after all and that make spiders some terrific ranged back up in many cases with spells that are almost unique to them compared to other evil creatures.