I get nests of baby spiders hatching in the greenhouse every year and setting up on the tomatoes. They were at the farm greenhouse last year too. The ones around the tomatoes are a really small spider - they never get large.
Based on their size, I reckon they are eating really small stuff - ie the darn mites, thrips and similar tiny tomato pests. The spiders come around early in the season and are a bit of a nuisance to work around, but they're doing a job. After a few weeks they just disappear from the scene - I presume when their work is done and there's not a pest population big enough to feed on.
Oakley when I first saw these spiders I had the same worry - are they spiders or mites. Eventually figured out they are too large to be mites, but just the right size to eat em instead.
