I use CV glass on both the Bessa and the M6 so I can't speak about Leica lenses but I have or have had the 15mm, 21mm, 25mm, 28mm, 35mm, 50mm, 75mm and 90mm lenses all at the same time.
I use wides a lot when travelling, particularly in Europe, for town and scenic shots. In those circumstances I use the 25mm a lot, the 21mm less so. But I recently sold the 25mm because the M6 has framelines only to 28mm and thereafter I need to use an accessory viewfinder (not a problem with the R4A). And I felt that the difference between 21/25/28 was just too little to justify three lenses in that range so I cut out the middle one even though I think it's a very useful focal length to have on its own.
If I didn't have such a selection already and had only (say) a 35, then I would definitely prefer to have the 25 and dispense with the 21 and 28 focal lengths. It's a more generally useful lens and shows little distortion. The 21 is great but you have to be a bit more careful about keeping the camera level to avoid the well-known distortion effects. The 28 isn't wide enough for a lot of situations and the 15 is almost a novelty lens and gets little use.
