I look at it this way. Anything I can do to simplify all the things I have to do to prepare the rig for a trip is well worth the investment. I like to make everything as easy as possible.
Any task that I need to do, I try to eliminate any hiccups that add time and effort to completion of that task. The GPS is not completely necessary, but it does (usually) simplify the task of navigating complex roads.
Tried Co-Pilot, but found it very buggy. Had Garmin for a long time, but there were issues with it, that added hiccups to the task of planning a trip and altering the route. So I recently got the TomTom Trucker 620. For me, it simplified route planning and route alterations. Very intuitive operation. It is another expense, but like I said, I like to make everything as easy as possible.
Like any change, there was a small learning curve, but once past that hurdle, I am very happy with the gps. I also use a Rand McNally trucker atlas that I use to verify routes are RV friendly.