If you kill off all the house spiders, you're more likely to get other pests that might be more hazardous. I killed off all my spiders, and the next week the ants came in. Killed the ants, and found a scorpion. If you start killing them off, you'll be stuck killing them forever.
That said, $400ish per year to not have to worry about stepping on a scorpion (wife almost did that) or kid finding a black widow or brown recluse might be worth it.