I think some sensors must attract dust more than others. I finally got round to looking at Craftysnappers dust test
and am more convinced than ever now.
In the nearly a year that I have had my D2X I have used the blower less than half a dozen times. While I limit lens changes on dusty rally fields, I am not too bothered otherwise. On a day out I may take 12-24, 55 macro, 105, 400 and converter, and will do several lens changes in the course of the day. After a trip to the coast I usually have to vac the sand out my bag, so those lens changes are not in still air conditions. Foxy would kill me for saying this but I quite often forget to turn power off - electronic cameras still getting my head round them.
( edit ) Ooops just done it again to check the macro lens for this post.
I think another factor also plays a significant part in the desireability of built in dust removal and it effectiveness and that is lens choice. My zooms are all short, so the barrel extension on maximum zoom ranges from 0 to 25mm. Most used is 12-24 at 0 ( environmentaly sealed behind the uv filter ) the 28-105 has 25mm full extension, I rarely zoom quickly anyway. All other lenses used are old fixed focal with the 55 macro having the greatest movement of elements. But as that requires turning the focus ring through around 300 deg, its not exactly quick. Certainly none of these could be described as mirror box dust pumps. Looking at some of the lens review pics on Photozone some 10x zooms seem to have a significant extension when zoomed out. So racking these in and out is bound to have a greater effect on any dust that may be in the mirror box.
Don