I agree that MITM proxies shouldn't be used on the public Internet and thus we shouldn't make it easier to do so, but what about the people who are already being MITMed? Is there another way to solve this problem or must we throw corporate Web users under the bus to save the public?
If someone can install a root cert onto your computer then you are already owned - there is no end to the other things they can do too. Call it a virus, call it an enterprise, but call it a day - you're owned and there is no in-charter policy this working group can enact to change the security level of that user for good or for bad..
The good news is not everyone is already owned and SSL helps those people today.