Another good question chadzat.
This is my 2 bobs worth, I expect some others will help as well.
There are of course many "reasons" why we do many things. Unfortunatley some are 'Myths" and some mostly correct. Remembering that you can't neccesarily apply one rule to all aircraft types.
The problem with most LIGHT retractable aircraft, when retracting the gear, is the rotational 'Gyroscopic" effect that the wheels have on the light support structure. Not discounting the wheel spining into the well idea, but this is more correct in my eyes. Both are good reasons to use the brakes. Unfortunately I don't belive that your friends information to be the most correct.
1. When either locked up or down the undercarriage leg has some form of strength within its supporting structure Ie it is locked with cams, pins, overcenters or other such engineering bits. Obviously when down the structure can support the weight of the aircraft plus the worst sort of touchdown. Light aircraft manafacturers do MOSTLY try to keep these structires to the minimum design strength to keep weight down.
2. When moving between up or down the sturcture is significantly weaker as the brace or locks are removed, during this time the structure is suseptable to damage by forces working against the drives. (electric motors , cogs, rods or Hyd fluid) A good example is that most light aircraft have a maximum airspeed for gear retraction as well as for extended. this is due to the load on the U/C motors ect, by airflow over the doors which do create thier own local forces.
3. The worst sort of Gyscopic effect is in aircraft where the wheels retract at right angles to the plane of rotation of the wheels. ie B55/58, A36, most piper retracts ect. The force used to move the rotating wheel is transfered to the U/C structure as a twisting moment. Over a long period of time these forces may/can/have caused damage to the internal components and structure.
4. As Atlas has pointed out (in his own elquant style) , the nose wheel does not have brakes, but the nose wheel in almost all light aircraft retract in the fore/aft plane, therefore the twisting moment is minimal on this structure. Also nose wheels are provided with "dampners" to prevent "Shimmy" during spin up and spin down.
The C210 wheels actualy move in several different planes of rotation during retraction. Just have your friend watch the wild gyrations of this type of cessna U/C. even do a test, watch one takeoff with brakes used to stop the wheels and then the other. let us know the results, if any.
Some one else may be able to provide more info, but I beleve that there have been several failures of the internal cog drive in this type of cessna undercarriage.
richo