I think to an extent this is churn, and I do disagree with the premise. I would suggest that a consistency of markup formats within the project matters more than copying-and-pasting into a third party tool (IIRC, GitHub copes with pasting in the rendered HTML, though one has less control).

This seems to be a loosing battle though, and README files have been changed to markdown for various reasons elsewhere, so I won't block this if there's desire to push ahead.

A