Think of a lens as part of a lens system. Then the question is, "What other lens(es) will I also carry, and how apart in focal length do I want them to be." If you want to also carry a 35, then a 28 may seem too close in focal length to bother with (I don't feel that way). I think the ideal set would be one where each focal length is 1.414 (the square root of 2) times the last one. That way, each next-shorter lens covers exactly twice the area of the next-longer one. An example of lenses spaced exactly this way would be 25-35-50-70-100. In practical Leica M terms, an approximation would be 24-35-50-75. An alternative set would be 21-28-40 (a good start, but there is no 56mm lens to fill the next step; though 75mm would fill the step after that.)