Those are some totally weird things to say to new riders. There is no reason that you shouldn’t practice and become proficient with slow race, cone weaves and U-turns. The rear brake is essential to all of these. Your tires are made for moving across the pavement at 10-15 miles per hour with 400+lbs of load. Your boot soles were not. Yes, I still drag my feet some (using Eon’s example- doing U-turns at gas stations), but if I am a year from now, I haven’t learned how to ride my bike properly and I’m doing something wrong.
Check out the video. You might also check out Capt. Crash’s videos too.