"Do I need a D800E if I have an M9 and Leica lenses?"
Because there is a far larger variety of lenses from fisheye to ultra-telephoto, including zoom, shift/tilt and macro lenses , available for an SLR camera design as opposed to a rangefinder camera. Likewise sometimes a small, quiet and precese camera and lens combination is the better choice of tools.
Now if you'd asked "Do I need a NEX-7 if I have an M9 and Leica lenses?" The choice becomes far more intriguing.