Does anyone else do this if their ds is caught short

[anotherdayyetanothernickname]

Yesterday ds was caught short in the car and needed a wee. I happened to have an old nappy (unused lol) and got him to wee onto it. Much nicer than peeing on the pavement and less likely to have errant splashes.
Any reason why I shouldn't do this - it seems so much easier than portable potties. Am I missing something (he is 3 and totally potty trained so no chance of any confusion)

Well,I'm sure its fine there was a thread the other day about mumsnetters peeing in nappies when caught short on long car journeys

I guess I wondered why more people don't do this too??