There are many different way to use a GPS. None of them are right or wrong.
Last couple of years I have participated (a lot) in some of the Tag O Rama games on ADVriders (I'm "1 Wheel Drive" there, FWIW). When heading out on a tag grab, I'll usually take a few minutes before leaving home to find the exact tag locations (I often go for more than one TOR tag in a day) using all of the resources of the intarwebs, then save them as Waypoints (aka favorites). With a general idea of which direction I need to go to find the location, I'll head out, and then at some point use the GPS to route me there.
I've hit some really fun new roads doing that by selecting the right combination of avoidances. I'll usually turn on avoid Interstates, and Highways, and when on the Vstrom I'll leave "unpaved roads" unchecked. If you then pick "shortest distance" (instead of shortest time) the GPS will send you down some interesting goat trails. On the FJR I usually avoid unpaved roads and use shortest time and still see some new roads quite frequently. Anyway, after finding the old tag I'll often just ride around aimlessly looking for something new and interesting to tag. I figure if the road I'm riding on is fun that other folks would probably like going there too.
But the other way that I use the GPS on the motorcycle is where I'll have planned out the entire route before leaving home. I don't seem to have any of the problems that snackbauer has with the route screwing things up, but that just may be because I've been doing that routing stuff longer and know more of the little tricks to developing a nice route.
One handy trick is to preview any of the roads that you are a bit unsure of in Google Maps, using either satellite view (zoomed way in) or street view where that is available. If the street view is available it is always a passable road, but can still be a dead end as the Google Van doesn't always know where it is going either, so you want to make sure the blue line goes all the way through. Sure, sometimes you still get surprised, but it isn't usually that big of a deal to go off the route and get back onto it again..