The point is don't trust anyone you don't know... I know people personally who's computer's were harmed by accepting a friend request on Facebook from an unknown person.
You're absolutely right. However, I think the biggest issue with facebook is the fact that many people keep their profiles in "private" mode. When you receive a friend request, sometimes people don't have pictures of themselves as their profile avatar. Sometimes it's someone you DO know, but whom you haven't seen in a while so you've forgotten about them. In order to figure this out, you have to "friend" them to see their full profile.
If I could make a suggestion: look at their mutual friends. If you don't have any mutual friends on facebook, chances are you don't know them. Most of the friends I have on facebook are ones whom I found through OTHER friends. Of course, if all else fails, send those people a message. Luckily, you don't have to be friends to send private messages to one another.