I think you'd want it, in my experience. Sure you could estimate based on the 28mm lines in the finder, IF you use an LTM-M adapter that calls up the 28mm lines. Given that you can (but may not need to) focus the lens through the Minolta finder, using the Voigtlander accessory finder means shifting your eye from one to the other. In the heat of taking multiple pictures, you could use the 28mm lines and just accept the extra brought in by the 21mm. Certainly, IMO, the Voigtlander finder is so nice and clear that it makes using the lens that much more enjoyable.