lightning LN has prisms and atomic tx chaining, so the following scenario is doable: I buy the latest album for $10, not knowing that $5 of it went to the original artist and $5 went to the merchant. Thus, the sender doesn't know how much money went to the receiver and how much went to one or more other folks. Which is good, that's none of his business. But XMR needlessly exposes that private info to him.