Erik,
Some zooms can be dust pumps as well , and it's more a function of the environment, care and switching frequency of your lenses than a question of zoom vs. prime.
Also the effectiveness of my dust shaking sensors is not 100%, and I still have to resort to an arctic butterfly every 1-3 month and sometimes even a wet cleaning. I also clean more often in the "macro season", not so much because of more dust or lens switches, but because I use smaller apertures more often.