You can find anyone, it's easy.
Removing the real names policy does away with that, and I would say seriously threaten's facebook's business/growth/future. If you don't know your old girlfriend or your cousin or your highschool bandmate's username, how are you going to get in touch with them?
I'm not saying that's a bad thing, or that I think facebook should have real names, but I'm saying that I seriously doubt they are going to change that policy because it'd cost them too much in the long term.
EDIT: People, I understand the arguments against real names. Facebook's counterargument is money. Which argument do you think they support and will listen to?
Downvoting me is crass, I'm not even stating a position, I'm just stating the obvious reasons for Facebook's continuing denial of a chosen-name policy.