This discussion has diverged into two situations :-
1. Retracting Flap at / just before touchdown or during the rollout
2. Fiddling with flap during the approach.
1. Retracting Flap at / just before touchdown or during the rollout
This is mentioned in the POH for some (SEP) aircraft for improved braking. I personally don't do it given the areas in which I fly, but I have done it. C182 with Manual Flap for example.
2. Fiddling with flap during the approach.
In the SEP's in is not in the POH, it is not a very good technique not to mention when you have the well known, tested methods we were all taught in our PPL's that actually work.
I'm not a test pilot - nor do I want to be.