My Dad's urologist/surgeon also left his urethra due to time spent on the table because of adhesions. The doctor said the margins on the urethra were clear with no indication of cancer spread, so he had no qualms about leaving it. I'm not sure why your Dad's doctor wants to put your Dad through surgery when there is no indication that there is an immediate need to remove the urethra. Was there indication of cancer there when your Dad had his RC? I know you said he had some positive lymph nodes. Were they in the urethra area?
Recently, my Dad has experienced some problems that has led the uro to think Dad needs his urethra removed. However, I think that a biopsy of the tissue should be done, at the very least, before anything is removed. Dad is certainly not anxious to go under the knife again any time soon. From what I've been told, the removal of the urethra is a pretty painful procedure for the patient. Right now, we're in "watchful waiting" mode.
If I were you, I would ask the doctor why he wants to remove the urethra and what would most likely be the results if you did not remove it. There are people who are able to have a certain type of continent diversion and still use their urethra to void the same way as before the bladder was removed, so not everyone has the urethra removed, although I think most do as part of the RC.