I agree with Gruber. Ambiguity is a feature. It allows every implementer (leanpub, github et al) to add their own idiocentricities while staying true to Gruber's "spirit" of Markdown.
Everyone is still free to implement their own version. But it seems like many of the people behind the biggest implementations would rather agree on a standard.
They still can. They can be as ambiguous as they want. They just wont be able to advertise it as compliant with "Standard Markdown".
I don't see a problem.
Can the same be achieved with a common core (spec) + 3rd party bolt-on extensions?