Glenn, to address your question further:
1. I feel that having multiple browsers implement the same prototype extension independently will expose ill-defined areas more quickly, and allow the extension and its conformance tests to converge more quickly.
2. Requiring a vendor prefix implies that there will be two nearly identical copies of the extension in the registry during its prototyping phase. Keeping them in sync will likely be my responsibility, and from a selfish perspective, I am opposed to having to do this work, which I consider useless.