Pull What, I entirely agree with you! It's hard to tell pilots to train things in planes, which the flight manual says are not approved, but there can be value, in a safe, supervised environment, for a new pilot to understand what happens if....
There is no excuse for "loosing control" after coming (or remaining) airborne with full flaps. Every certified GA aircraft has demonstrated climb in that configuration, as well as the ability to rapidly go from full to no flaps without loss of control. You might not get the expected climb performance with full flap out, but it will safely fly, and climb at least a little.
When I flight test, where I am expecting changed control effectiveness, I sometimes will take off with full flaps, to get the aircraft airborne at the lowest possible airspeed. This way I can evaluate the control effectiveness before a "flight" away from the runway is committed. I do this in accordance with a flight test plan though, so deviating from the flight manual is permitted.
Many aircraft have amazing capability beyond what the flight manual says. I recall seeing a Cessna publication titled "Getting the maximum performance form the Cessna 150". I never had the opportunity to read it, and now Cessna offers no acknowledgement of it's existence! Liability, I'm sure, but would I ever like to read what they wrote back in the day.....