That works for most swaps. But I know there are some swaps out there where using a padded envelope is just too much. When I helped co-organize the Yarn for Rapunzel Swap, [as far as I can remember] we all used regular envelopes since we were sending about 10 pieces of yarn.
But I do agree that padded envelopes and boxes are ideal for most other swaps. I would just make it a suggestion, and not a requirement since it doesn't always apply to all swaps.