Shouldn't it negotiate with the Source using its Cable Identifier and simply provide 60W (20V @ 3A) maximum to the MBP? So a slow charge, but not dangerous?

Has anyone tested this scenario in real life? I'd imagine that if the machine is idle, 60W would at least provide break-even, and in sleep, should be able to charge it. But that's assuming that the identity negotiation works properly with this hardware combination.