Well,
legality!
If you are forced to use higher speed than maximum allowed for app category and yet you choose to follow lower cat procedures/minima, you have extreme chance of getting out of protected area and seriously compromising your obstacle clearance.
The aeroplane is perfectly uncaring whether it's certified according to FAA, EASA, DGAC or whoever. At given speed and given bank angle it will have exactly the same turn radius in coordinated turn, no matter whether it was designed by Henneman, Sukhoi or Howard.
Seems that EK Gann's lesson of rule books being only paper didn't make it into ATPL multiple choice questionnaire database.