I don't want to manage a "business relationship" with every shop I buy something from. That's way too much effort. There can certainly be cases where a more complicated relationship is created by bootstrapping off BIP70, perhaps with an extension, but nailing the ordinary buyer-to-seller relationship seems like a good scope for BIP70 for now.
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.