One of my favorite instances of Dean' use of "Sammy" fits really well into this analysis. It's in Asylum after the gun confrontation, after Dean knocks Sam out he then pats him on the side and says "Sorry, Sammy." It's a point when Dean is in serious big-brother mode, having just had to "take care" (heh) of Sam. Sam is not an equal at this point, but had to be sidelined for his own good.