Daniel, this kind of touches on some other threads related ot your RZ experience, but one of the major reasons for me to shoot MF is the availability of interchangeable backs. Being able to go from one format to another, or to shoot polaroid proofs or type 55 (glorious stuff), and then go quickly from one film to another... or perhaps to a digital back... those capabilities are one of the things that most distinguishes small format from MF. Very few 35mm cameras can switch films mid roll. And very few can generate reasonable polaroid proofs.
I could care less about what this or that lens can resolve, that is primarily a small format concern. If I want to record the head of the pin from 100 meters away I know exactly how to achieve it, and I don't have to pay a lot of money to make it happen. But it simply doesn't interest me, artistically speaking. Another thing, we often hear about how wonderful the bokeh of a Leica or CZ lens is, but look, you have far more control over in-focus and OOF elements with MF and LF field/view cameras. So that is another aspect of MF/LF that hardly exists in small format (except for pricey t&s lenses with limited capabilities and a few digital rigs now used by architecture shooters). After working with a field or view camera for a while, the use of bokeh in 35mm starts to look, well, one-dimensional and obvious. (There, I have probably just offended 75% of the site, sorry for that!)
Summary: there are plenty of reasons to go to MF or LF besides grain and resolution and how big the print can go.