I have a rule on Facebook that I follow pretty strictly.
If I have never spoken to you, either in email, chat or in person, prior to your friend request I will not friend you.
There are more than a few people who are miffed over this, as they say I went to high school with them, or, they are my 8th cousin, or they know some guy I know. But I don't think I am being unreasonable.
I think these people just want more friends in their list. As others have said, it's a status thing.