I do think those children who can be toilet trained by that age should be. I don't believe it should be if a child has a once in a blue moon accident however.
We are told to send in spares for DS1 in P1 (Scotland doesn't have reception) he's only needed to change when covered in mud!
DS2 goes to private nursery with lots of changes he is potty trained but he's also not 3 yet and some days he can have accidents (especially when tired) I would worry about school nurseries doing this!
I agree the absorbency of disposables has contributed to the delay though.
I was called a middle class hippy for using re-usables with both my kids....reality was I hated the smell of disposable nappies!