I personally love the DA 10-17. I do change lenses in the field and the 10-17 is a great walkabout together with 28-75. Would you rather have the 300++$ that it will net you on the used market or the fun hours you can spend with it? My friends get a kick everytime I use this lens to either make them look huge (close focus) or small (shot from above). It's one of my most versatile if not my most versatile lens, imo, since it does four things at once without any add ons (wide angle, fisheye, portrait, macro). If it's a fast aperture zoom and goes to 23mm rectilinear while keeping IQ.... that would make it a perfect lens for me

(hey, I can dream right?). If you don't use it that much for whatever reason, you should sell it though.
From the time I got it I tried to "take my time" knowing it, since from what I see people seem to use it for a little while to get their little kick out of it, then sell it. I'm glad that I took my time because otherwise I probably wouldn't have used the lens in the many ways that it can be used.
By the way how do you find DA 18-250 compared with the sigma 17-70? Other than the Pentax - Sigma color differences?