Where I live, spiders might get large, but they aren't typically aggressive. I have woods in back of my property as well I burn wood for heat, so I import a number of pests in the house, namely male garden spiders and wolf spiders. To people in warmer climates, these are small time, but the two examples below are a wee bit scary because their size is a bit unusual in these parts and the females that grow huge under the roof are out of the way and nonagressive. I'll use a spoiler tag to keep arachnophobes from being too unsettled:
I prefer to let them be, even if they're inefficient at killing mosquitos because they're interesting, but it isn't like you're going to cause the extinction of an entire species, and I wouldn't feel terrible about killing one because it's not like they're in danger of extinction, and they're more effective than Raid for killing silverfish, which I now have in my home and think that fumigating is going to cause more harm than good.
I also have a small population of zebra spiders [http://en.wikipedia.org/wiki/Zebra_spider]. Those are awesome entertainment because they can jump small distances. I wished I could import those into my house because I'm sure they'd cause an extinction of ants and silverfish.