Think about it. The spiders seem little more than arrow fodder to the elves, so they probably are used to fighting them off from the fringes of their forest quite regularly. I feel like they wouldn't want their toxic bodies decomposing in the forest, so they would at least move them. If they are going to move them, they may as well harvest them as well. They could use their copious amounts of silk to create cloths and strings and whatnot with near-mystical properties. The only reason we haven't made spider silk cloth mainstream nowadays is because it's hard to harvest, but with giant spiders this problem is eliminated. It may very well explain some of the more impressive feats of elvish clothing seen in the books.