I'm not sure why people always say this. There's an "Ignore" button for friend requests, or are people just that deathly afraid of offending people that they accept every friend request that comes their way? If someone I had no time for in high school tries to friend me I just click "ignore" and go on my way. It's not like they get a notification saying "Owen ____ has ignored your friend request!"
And if you don't have all your privacy settings on maximum that's just asking for trouble.
It's just that you didn't used to have to even click a button to ignore these people. Merely being on the internet used to be enough. Now we have to do something to avoid them, just like in real life. Sure, you can cross the street and pretend you're talking on your cell phone and don't see them, but it was nicer when they were living in another state and it was just never an issue.
Interesting you bring up privacy settings because my experience is that the illusion of privacy on Facebook is what persuades some of those people to participate. But even if you trust Facebook that much, really - if you say something to a room filled with twenty or however many of your friends, you are so positive that none of them will ever repeat it? Someone was just telling me a story yesterday of how she noticed she'd been unfriended by person A, and person B told her "oh yeah, A just posted an update that said she'd unfriended all the bitches on her friends list."