You would only use this ZI SW for wide angles.
Like Hasselblad SWC, Alpa 12 TC, ZI SW is "blind" for the distance. When I am using my ZM21 2.8 on my Leica M3, I rarely focus with the rangefinder. Looking at the depth of view readings seems to be more important to do before you take a pic. As you know, there is no coupling of rangefinder in ZM15 2.8, so using this lens in a camera with no rangefinder is more sensible.
I would prefer to use Leica MD, MDa or M1 on ZM15 2.8 for their excellent shutters. For those who prefers light weight, ZI SW is the choice.