IIRC Protouser was my idea for dealing with the "new account problem". The idea was that when someone registers they create a protouser. When they actually log in for the first time the protouser gets promoted to a real user. If protouser accounts were used after a certain time they were to be automatically purged.

As I recall tye didnt like the implementation, and since the code involved PM module changes which I tend to leave to tye the patches were never applied. tye had some other idea for how it was to work, but I dont know if it ever went anywhere.