Aren't spiders developed and employed by the search engine companies? In that case, their purpose would seem to be to make the search engine (results) more inclusive and up-to-date. Getting listed is a positive effect from the point of view of the web master of any given site, but the purpose of the spider is to serve the needs of the search engines. Semantics, to be sure, but hey - look what site we're on!

"I don't know which is worse: ignorance or apathy. And, frankly, I don't care." - Anonymous