Slightly off-topic, but if it wasn't for the rapid blockchain growth and my procrastination in fixing Armory's scalability issues, I'd have Multisig-with-P2SH implemented already. The problem is not the features being useless or unwanted, it's that there's such a shortage of human capital to make progress in this part of the Bitcoin development world. There's so few environments where such a feature could be implemented, and so many other priorities of the few people developing those environments.
However, this particular change is trivial to implement, and dramtically simplifies implementation of any kind of offline signing device. There's no doubt it would be used immediately by Armory and all the HW wallet developers.
If it's so trivial to implement then where is your proposed implementation and BIP? It could be re-implemented fifty times through rejections and objections and still be less work that the true work associated with all the testing, wallet implementations, and alt-implementations associated with this idea.
Anyway, it doesn't simplify offline signing all that much, it just makes it possible with more limited hardware. The actual code is basically the same in both cases. Heck, the design effort required for the hardware isn't much different in most cases: moderately fast USB interfaces aren't a big deal these days and come pre-packaged.
This also encourages the design of really limited hardware wallets that don't support the payment protocol: if you don't know who you are paying, all you've done is limited the rate that your funds can be stolen a bit. Heck, on that basis I think I'd actually NACK such a patch myself...