This seems easy to implement. What are the good reasons not to?
Jay has this on his Kick Starter list
Global rankings are extremely easy to manipulate. Players can create their own games, they can join games with multiple friends, or they can play with multiple accounts.
When I add the friends system I would like to have a leaderboard that shows your friends rankings.
I think its more interesting than seeing some dude at the top of the leaderboard that you don’t know and have no hope of competing against.