At the risk of being beset upon by a throng of angry instructors, it has been said on PPRuNe, and I am tending to agree, that the training organizations are building upon a "heritage of inexperience". When the capabilities designed into the aircraft, are being supressed and under utilized during training, those who come to conduct training (new instructors) will do so based upon that diluted training, which they themselves received. This will continue that heritage to the generations of pilot to come.
I am frequently amazed to experience what training is comprized of now, relative to what I was formally taught 35 years ago. Flap use and sideslipping are certainly two of these things. Pilots seem to be scared of flap use, for no reason I can see. If flying with fully extended flaps requires more skill, then learn and practice that skill! I have certainly demonstrated full flap techinques to newer pilots, sometimes coupled with STOL kits, and have literally left them speachless. These were not dramatic demonstrations, those newer pilots were just unfamiliar with what to expect. I have had air traffic controllers ask if the plane I just landed had a STOL kit, and often I would reply "no", just the regular version. They obviously did not see them landed short very often.
Soft field landing techniques tend toward less wear and tear on the plane, lower speeds (= less risk), and require greater flap settings. Short field landings are similarly the latter two, though can be a bit rougher on the plane sometimes. I cannot think of a good reason that most landings a pilot would do, where he/she had the option of type of landing, would not be a practice soft or short field technique. Perhaps it is just the exmple which has been set for them during training.
Re the sideslipping prohibition - some Cessnas are placarded as such
Not "Prohibited", just "Avoid". In the certification world there is a difference. If "prohibited" were
required becasue of the way the plane handled in that configuration, it would not be certified at all. It is not possible to "prohibit" away a certification requirement, and sideslips in all configurations are a certification requirement.
Thing,
The 40 flap landing in a Cessna will result in a shorter length of time in the flare, before either you settle to the ground (gently, or "arrival"), or have to add some power. Next time you have lots of runway, and you are comfortale with full flaps for the conditions, try easing some power back in as you begin your flare. It will prolong the flare a little, and soften your contact with the ground. Once your main wheels touch, hold the attitude so the nose wheel stays off as you close the throttle. Keep holding the nosewheel off for the whole rollout. You'll still have a gentle, and pleasing landing, and it will be happily short too!