I have no particular objection to friend requests from people I don't know, especially if we share an interest or have friends in common. It is, potentially, a way of 'meeting' people in the same way as interacting on Mudcat or TheSession or Chiff&fipple.
And just because someone someone sends a friend request doesn't mean that I have to accept it. In fact, I ignore/decline most from people I don't actually know.
There are sinister/irrelevant/nuisance friend requests, but one is free to ignore them, just as one doesn't have to talk to the drunk in the bar who tries to strike up a conversation with anyone who goes near.
Some people just collect Facebook friends. I know people who have 500+ friends from all over the world. Don't see the point myself, but I expect there is nothing more sinister about most of the friend collectors than a bit of shallow competitiveness or insecurity. Certainly some kids seem to compete to get the most friends.