I’m a big fan of the ‘pm-as-draft-message’ approach, and have had a number of these warnings too. Perhaps I have been too accepting of the status quo. 
I don’t know how popular this workflow is, so we may be in the minority, but we could make this a Contribute > Feature request?
Alternatively, on a more minimal scale, a Contribute > UX approach could be to include more information in the warning message (eg. “Body is too similar to what you recently posted. Try again in %{unique posts mins}”). This may at least emphasise that it’s time-based and only a temporary restriction (though possibly undermining the intent?).