Back to the OP's question I decided that I wanted the *istD as a K10D backup. The main reasons are, that it is basically laid out like the K10D for shooting controls. Thumb wheels etc. Also that it can work in TTL and P TTL, better for IR and low light ISO.
So when I switch bodies, say at a wedding where one camera has a zoom and the other has a fast prime, I don't have to think about the controls very much. I have a K110D (that was given to my son) and it's a great camera but in a hurry I was finding myself looking for a control button/setting that wasn't there.
TTL. I have a pair of AF400T's left from my film gear that are the best flashes ever built by Pentax. Since the K10D can't support them I wanted a body that would. I'm looking forward to being able to use these excellent units again. Might even look for a AF080 as well for better macro shooting.
Of course the new body in January will have TTL now that I bought a used *istD.....