I was taught this during a UK PPL in the 1970's. Like the post above it was explained to me it was in case of uneven flap deployment causing a roll. When I instructed in the USA and enquired as to whether they had a similar policy I was told it was a quirk of the UK and that Piper and Cessna had no such opinion on the matter. Since then I have had no qualms on extending flaps in turns in GA aircraft.