If you have one portable nav stolen the cost will be more than the depreciation of a built in unit plus you will have to deal with a broken window or some other damage.

With the built in unit, I just turn the car off and leave. There is still a risk of break in to steal the built in unit, but it is much less than a portable.

I also got mine because of the energy screen and would have probably paid 1/2 the price if I could have gotten it without the nav, but now that I have the navigation I find I use it for places I know where they are just for the time estimates even though they are not really accurate.

Almost all options pay off poorly on resale, and the best car to buy for resale is the base modle with no options.