Loyd,
The spiders in the "Who's Online" list on the Portal page are automated computers that "crawl the Web" to gather information that a search engine like Google will use to determine what info can be found on what page. They build an index with that knowledge so they can give the most relevant answers to searches that people submit. So if you want search engines to give accurate results for your searches, spiders are necessary for that.
The spiders can't see in your Profile or your PM messages. But anything that is in a public post can be seen by a spider or any Internet user anywhere.
