One way of achieving multi-path payments in lightning is by having the sender use the hash sent by the receiver to send multiple payments along different paths. The mailing list entry proposing atomic multi-path payments describes a downside where
the sender fails to actually satisfy all the payment flows, then the receiver can still just pull the monies (and possibly not disperse a service, or w/e).
But if I understand correctly, if the receiver pulls any flow before all paths are complete, the sender will still end up with the invoice and pre-image which they can use to prove the payment went through, and if the receiver refuses to provide some service, the sender has the same recourse as in the case where the payment was a single-path payment paid in full. Why then, is it necessary to have this atomic multi-path scheme, which makes use of unused bits in the lightning protocol that could potentially be used for something else?