Um silly question, but it looks like you have 3 GPS's. The stock and two others, can I ask what the other two are for?
I run Long Distance Endurance Rallies. Think of a motorcycle scavenger hunt, where all of the US and Canada is fair game. Some of these run 12 or 24 hours, and some run 3, 5, 7, or 11 days.
The trick to it all is to maximize the time/speed/distance equation. So multiple GPS units give me lots of data that I can use to decide where to go, which way to take, and how long it'll take me to get there.
I prefer the Lowrance GPS units because of the large, sunlight-readabe screens, and the fact that I can put 10 data slots on the main map screen. So I always know time and distance to next turn, time and distance to next waypoint, time and distance to the finish, and arrival times at these places.
Having two not only adds redundancy, but it allows me to run multiple route scenarios, and instantly see the difference in time and mileage. That allows me to make good decisions to maximize my points, and my final position relative to other riders.
The Nav IV is useful as it has the very latest maps, plus it interfaces to the bike to help track fuel range. But it is smaller and farther away and it doesn't have the data capabilities that I need, so it doesn't work as a primary Rallying GPS.
For just tooling around with friends, or crossing states just for the fun of it, it word just fine. But for what I'm doing, I just need more.