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I don't think there is a setting for treating spiders as anything but anonymous. You really really really don't want them as anything else. Search results would be useless since a user clicking a link would come in as anonymous anyway. But security is the biggest reason.
You do however set-up your Meta tags etc. to best reflect your site. Personally I set-up a robots.txt file so that spiders (good spiders) don't index portions of my site that would not be beneficial or would go stale quickly.