It is not more effort than an auto remembered call-in phone number. You delete if you do not care. The difference however is that it would be a clean protocol for repeated payments in both directions for whatever reason, where "refund" is and "payment" are not special compared to "1st installment", "overpayed back" or "tip" or whatever extra charge arises later.

I think that'd be too abstract. The purpose of the refund field is that so if/when you receive a payment there, the wallet UI can do something intelligent, like show you in your transactions list that a certain payment was refunded using language the user will understand. If it's modelled at the protocol level without that then it makes producing good UI's harder.