In all my years of scouting I've never known of a youth attempting to earn a merit badge who wasn't in scouts. It also seems that the advancements person could easily determine that the young boy who turned in the unsigned blue card isn't registered in the troop, no need to have the SM pre-determine this. That said I've know numerous troops who thought a boy was registered only to find at council that somehow the application hadn't made it in yet.
Instead, I believe the signature is part of what forces the scout and SM to see and get to know each other. It gives them something to break th