I think you've been fed some bad info there, D-type asteroids might have some water content, but thats not really proven yet, and we don't know how much. Soft landing one on the martian surface would take a really phenomenal amount of power. Why go to all that bother when mars has massive amounts of water ice on or near the surface anyway?
My two cents is that the first step on the road to colonising mars is getting something to live on the surface, and I think thats a great goal in its own right. I think we should be looking at small things to get the ball rolling, rather than leaping to massive astro engineering projects like full terraforming.