It's a matter of combination, camera + lens together. If I want the best result I can get, yes I would love to have a combination of a zeiss 21mm with a FF camera. The DA 15mm limited even though you say that is is comparable to a zeiss 21mm on almost same cameras with APS-C sensors, cannot be used on a FF camera. And the Zeiss in not just slightly better is much better at F/4 than the DA 15mm, thus if you want to get one you pay the double price.

As for mine DA 15mm and as I've used it on my Pentax K-r, the lens is soft at borders until f/11 (maybe it's my copy - I don't know), while the zeiss is usefull from f/5.6 (according to photozone test) and onwards. That's a great advantage for low light situations and gives more flexibility, not to mention that the Zeiss can be used both on a FF or on an cropped sensor camera.

Maybe the DA 15mm could be able to give the same optical results (at least to my eyes) like the Zeiss or the Nikon 16-35mm G VR if Pentax had a camera that would take advantage of it. But Pentax hasn't that camera yet. As far as I have seen there is no competition to what a Zeiss 21mm can deliver on a D800 or on a D600.

For example these are my shots with the DA 15mm on my K-r:
http://500px.com/photo/36642174
http://500px.com/photo/35546190
http://500px.com/photo/35607682

And yet they cannot compare to shots like these:
http://500px.com/photo/44868382
http://500px.com/photo/37616966
http://500px.com/photo/21838995

The resolving power of the FF cameras is umatched. The resolution and the colors are splendid. How can a DA 15mm limited can deliver such quality on the current Pentax camera line?
That is why a FF camera is necessary for Pentax in addition to something like the DA 15mm (a DFA 20mm maybe) to provide such clarity and quality.

And this the biggest contradiction in Pentax philosophy. Number 1 reason they put the “Tough construction for stress-free shooting in all places and all kinds of weather”.

So what kind of photographers the Pentax brand sets as a target and possible buyers? Those people who will be out to sea or on the top of a mountain, the action shooters, the wild life shooters, the landscapers.

An APS-C sensor inside a sturdy lightweight camera suits the needs of wildlife shooters, the fast FPS suits the need of action shooters like those who do skiing for example, but an APS-C sensor with the current Pentax lenses does not meet the requirements of landscape users who want the best detail, dynamic range, color rendition, ISO capabilities of a FF machine. It’s like comparing apples to oranges.

For example search in 500px.com for Nikon 16-35mm VR or Zeiss Distagon 21mm or even the Canon 17-40L. My sense is that the above lenses on a FF camera outperform the results I get from my SMC DA 15mm. The new Nikon 18-35mm G also outperforms the DA 15mm in its equivalent length. Why shouldn’t I want a FF camera like the Nikon D 600 when I know that I will have better results with it than using a K-5IIs with my DA 15mm?

And it’s not that the D600 is a much better camera from the K-5IIs, it is just that on Nikon D600 we can put better glass that will utilize the power of that Nikon FF to produce astonishing results.

So why to have a “Tough construction for stress-free shooting in all places and all kinds of weather” when I cannot be happy with the photographic results that I can get from it? Is it so difficult for those guys there at Ricoh management to understand that a landscaper needs not only a tough camera but also some glass that will provide the best results ha can get with that camera. Do they believe that the HD technology is enough?

If Pentaxians had the choice to use a Tokina 11-16mm then everybody would be happy, because even the Sigma 10-20mm f/3.5 which can be used on Pentax cannot reach in terms of sharpness and color rendition the Nikon 16-35mm VR. But the Tokina doesn’t exist on Pentax mount maybe because that would mean a direct opposition against the DA 15mm or the DA 12-24mm, so what choices Pentax brand leave to me that I shoot landscapes other than to jump from ship?