A full flap takeoff serves nearly no useful purpose, other than to confirm that a balked landing climb is possible. Do not attempt - not an approved technique!
A full flap take off serves a very useful purpose in demonstrating to a student what can happen if they ever do take off with full flap accidentally selected down.
I can remember a Cessna 172 taking off at High Wycombe with full flap that the pilot had obviously left down after his pre take off checks. The pilot and 3 pax were killed when the pilot lost control shortly after take off during the attempted climb out. I am sure if he had experienced a full flap take off at some stage he would have immeadiately recognised the problem and have been able to take corrective action and thats what professional flight training is about, training for the incident that may never happen.
Who was it that said
The emergencies you train for almost never happen. It's the one you don't train for that kills you.