Yeah, Kirby has mentioned that before.
Back to the subject: I think the choice depends on the user more than anything else. I have used both dedicated motorcycle GPSs as well as automotive GPSs as secondary units, both covered in plastic baggies as well as inside a Ram Aqua mount. My take is that if you are counting on the GPS and cannot tolerate the time it may take to tinker with the GPS then the weatherproof version is almost required. The automotive types have displays that are not as bright as the Zumos, at least in my experience, and adding anything to shield them from the elements only makes it worse. If the GPS is being casually used and the person using it is not dependant on being able to see the unit clearly or hear the directions, then an automotive unit is probably fine.
Another thing to consider is that the motorcycle units, at least the Zumos, all support bluetooth and MP3 players which makes them much more useful if used with a motorcycle comm. system. They integrate nicely and allow easy and fast control and display of both the MP3 player and cell phone directly from the screen. Auto units are tending away from these functions and I do not think there are any current units that have both MP3 player and blue tooth capabilities.
All of that said, motorcycle GPSs are a terrible value IMO due to their high price. I own a Zumo 550 and use it all the time but I cannot justify the high price really.
Brian
Some people are easily amused.