I have to admit since I obtained the DA 20-40mm and am using it exclusively on my KP where it goes like a hand in a glove, not other bodies, I don't often put my DA 21mm to use. The only time I now use the 21mm is when I want to go extra-compact with one single lens, or maybe just the DA 40mm LTD as well, or possibly also the small DA 15mm LTD.
The DA 20-40mm LTD might also work that that way with you. But it would not replace your FA 31mm, which is a fine fast prime. If you need to avoid the field curvature at the long end, you can always slap on the tiny DA 40mm, If you shoot with another body than the KP its compactness might not be as advantageous.
The thing is, it is a zoom lens and does what zoom lens do- allow instant change of FL without having to take time to change lenses. And this with very fine quality along with being very compact for a zoom lens, and very well built with WR construction. So if you are considering a fine quality compact zoom lens having WR, this one could be useful.
As a kit, i often use it on my KP in conjunction with the DA 15mm and DA 70mm Limiteds, or the FA 77mm if I need the faster aperture and more reach. This kit is very compact with great quality.
You can visualize its quality in the still-life shot on the Imaging Resource test review of it. You should easily find that by doing a google search for that lens + review.
Last edited by mikesbike; 04-04-2019 at 08:11 PM.