He argued that since the signer(s) could have signed an arbitrary transaction instead, being able to delegate is strictly less powerful. However, Wuille responded that this seems to not be a concern for typical P2SH uses, but in many cases, not all stakeholders in a transaction will hold one of the private keys required to sign. They would want a guarantee that the original script is followed as promised.While flags typically wouldn't have a meaningful effect for funds from non-P2SH addresses, the concern is providing a transparent guarantee that the original script is followed as promised. Moving funds to an address with new rules that you'd otherwise graftroot in isn't the concern; it's ensuring that the original script is followed. The ability to move funds comes after following the original script and is unrelated.