Well Dave, it sounds like you want to be convinced, so here goes!
I think the purchase of a GPS (Garmin III+) was the second best investment for my South America trip, after my bike (G/S). I found the GPS invaluable for navigating through large cities when you can't read the street signs (or there are no signs!). It's easy to get turned around in the cities, but with the GPS you can always tell if you're heading in the right direction. I've also navigated with a compass, and it's not as easy because the compass only tells you where you're currently heading, while the GPS shows you where you've been as well as where you're going. Using this you can track a route through the city, correlating the streets on the GPS to your map. Another trick is to always store your hotel waypoint in the GPS to make it easier to get back later (a lesson learned the hard way after riding in circles around the streets of Santiago one evening!)
The GPS was also useful in showing me if I was headed toward the next town or city, and if I was on the right road. At one point in Chile, I thought I was on the coast road, but consulting the GPS I could see that I was about 20 kms inland from the coast and on a parallel road. Try that with a compass and a map!
I found the Garmin Worldmap adequate for navigating in South America - pretty good detail and not too many errors. However, in other parts of the world (Turkey for one) the Worldmap is way off, either intentially or because of bad data. Just remember that the GPS map display is only as good as the data it's operating from.
My advice is to get a good GPS and mount (Touratech), wire it into the bike, and carry a good supply of maps with a compass.