quid,
What I mean by unstable is that there's a lot of thrust lever activity for the poor old FEO. From personal experience in B707 and B747 classic it is much harder to stay on speed (manual thrust) at MRC rather than LRC or MCC.
As you say, MRC is not Vmd.
It needs to be clearly understood what we're trying to achieve - MRC is for most miles, Vmd is for most time.