People with experience with other navigation software have noted that other products, like the iGuidance / iNav line, are far better for actual, in car navigation use.
Streets and Trips, on the other hand, are far better at giving finer detail, for planning trips, etc.
It would be extremely simple to devise a software "toggle" that:
A) Provide an interface optimized for "in car" navigation use
A.1) iGuidance does this with a special, larger screen
(that support touchscreens) for netbooks.
B) Provide the traditional S&T interface
Want to get real fancy?
Have an interface that is optimized for "walking around use" on small screen handhelds with or without touchscreens.