The script subsystem only requires the tx you are signing.

The point is more succinct - avoid extending the Bitcoin standard.

I would also like a SIGHASH flag where you can specify a specific output to sign (so you can specify change address with pledges for SIGHASH_ANYONECANPAY), but I don't think it's a good idea to change Bitcoin's current design. More dependencies and execution paths = poor design = insecure code. We should be taking things out, not putting in.