Its just away to get away from the formality of "Dr Lastname"
If someone like Jack Newman isnt refered to as Dr Newman far be it for me to be refered to formally.
Somehow my using just Rob doesnt get people to drop the Dr so I can at least get them to drop the lastname stuff.
As to other doctors I dont know why people dont refer to Dr Nancy (Wight) or Dr Kathy (Auerbach)
An aside I once asked one of my interenship mentors, a doctor with a PhD,DO,and MD, why he had a DO and MD to which he replied, "Youre forgetting about the tough one".