Where I am we have a huge amount of wolf spiders and parson spiders. Both of them have (from what I've heard) pretty painful bites[1] so I like to keep them out of the house but I almost never kill them. I usually just let them outside.

I'm fairly sure it's not a coincidence that I almost never see a mosquito even though I live near a pond.

Now if only we could train them to eat deer ticks...

[1] Like a bad bee sting. Not dangerous unless you are allergic (possibly?).