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Tsotne Well,that might be one line of reasoning. My point is that I am very clear which direction I want my PhD to take and a generic MBA is not going to be of any value add to me. Also most of the PhD in mathematical finance programs have coursework requirements which seem to include most of the MFE coursework.
Having said all that, I have also applied for PhD programs directly where I am not too sure if I will be accepted as I don't have a masters degree nor any major publications to my name. Hence MFE is my first step. Likewise, MBA might be the first step for someone else, I do know a lot of people who have followed your path and been successful.