Iddo, you are right that there are some scenarios in which we would lose some expressive power of contracts. However, we are not aware of any known contracts which would be affected this way.

On the other hand, our change has a significant advantage for the contracts over the proposed softfork, namely it allows to sign a transaction B which redeems A before A is signed. In this case we cannot compute TXID(A) (and therefore create the transaction B) because it depends on the signature on A and the sender of A is always able to create multiple valid signatures on it. Usage of NTXID would allow to create and sign a chain of transactions with input scripts, which will be determined later. We believe it substantially enhances the possibilities of designing contracts.