Of course it doesn't depend on the secrecy of the userID. But having others not know the userID is good peace of mind. It also means that someone doesn't call the service on the phone saying, hey, it's me and identify themselves by my userid.

No one else read Mat Honan's story? http://www.wired.com/gadgetlab/2012/08/apple-amazon-mat-honan-hacking/

I sweat I spend more time correcting auto-correct sometimes than I do my own typos.