How would this be useful? I usually select the music I want to hear, then input my nav destination or switch to the nav view while I'm driving. I wouldn't want SD to switch back from the nav view unless I told it to do so. I think most other users use SD in a similar fashion...but I could be incorrect. I think that automatically switching back to the nav view while the car is in motion might be more useful than switching back to the main screen.
The option to automactially switch back to ANY user defined module after a period of inactivity might be the most useful feature of all...and would seem very easy to implement as a digital mod or as a feature of SD itself.