Lack of a really wide lens is what has kept me out of MFD. Used to use the 65 and 75 when we shot 4x5 film. The Sigma is far from perfect, but it works so well for my needs. And quite frankly none of my clients would notice the difference if I could shoot MFD. Most of this stuff never goes beyond 11x17 on press.
I don't understand, an IQ260, HCam or Alpa FPS, and a 17mm TS or the new Canon Fisheye are great ultra-wide options in medium format.
Or a Rodenstock 23HR. The equivalent to a 13mm or 15mm FF dSLR lens (depending on if using vertical, horizontal, or diagonal for comparison).
Resolution (size of print) is only one image quality advantage of medium format. It's easy to focus on because there is an easy-to-reference number. But the visual rendering (color, tonality, file-malleability, dimensionality, microcontrast) are all different.