Originally posted by MrB1 it to having no desire to use monster lenses with relatively short focal lengths on my KP
A lot depends on the way we get around and what else we carry, and what we like to shoot. My answer has always been both types have something of their own for advantages that are very important under certain circumstances. So I own both types, the HD DA 20-40mm Ltd, and not the new DA* 16-50mm PLM, but hanging on to both the 20-40 and for me my very satisfactory Sigma 17-50mm f/2.8 EX DC HSM lenses. For me they serve very different purposes. I often need very compact carrying. My usual kit for this are my KP with the HD-DA 20-40mm, HD DA 15mm, and HD DA 70mm Ltds. This is also in keeping with an obvious design goal of the KP- compactness with very high quality and versatile capability. But sometimes my needs for compactness are not as great, and a need to change FL rapidly (a zoom lens) along with moving subjects and/or reduced lighting being the conditions to be dealt with. My fast Sigma zoom lens is then just the thing. It can also be combined very effectively with my Pentax DA* 50-135mm f/2.8 for an excellent fast 2-zoom lens combo. This excellent, fast tele zoom lens is amazingly compact for what it is. Sandy is right, this combo is sooo much more convenient than having the DFA 24-70mm and DFA 70-200mm f/2.8 combo system on my K-1 II, lenses which I passed on.
When a fast f/2.8 constant-aperture zoom lens is what is needed, nothing else will do to meet this need.
I am happy to have both, very useful options.