The consensus seems to be that no one thinks it's worth doing this. I still think it makes sense to do as one of those latent upgrades that goes in now but isn't active until 12 months from now. But as you can see from the discussion some people are very focused on overhauling the SIGHASH system entirely, and don't think it's worth the effort to do this small change. I disagree, but what can I do?
The consensus unlikely includes those developing wallets. It is the single most dangerous "feature" I came across while implementing Bitcoin that one is not able to validate the fee of a transaction on its own but need to retrieve its referenced predecessors. We should do this.
For reference, I specifically recall retep talking to Gavin about it on IRC shortly after his last post, and Gavin agreed that modifying the SIGHASH system was a bad idea. I didn't feel like pressing it, at the time, though.
That's not to say that this can't be done or that Gavin and the other devs aren't persuadable. But it would mean mobilizing support for it and presenting a strong case to those that are against it. If the system was built with feature from the start, we'd all be happy and no one would have an issue with it. It's more about convincing folks that the benefit is worth the trouble of making the change. As you said... the people who most strongly support this are the ones actually developing software that has to deal with the megabytes of supporting transactions just to verify a few bytes.