How about while waiting in the airport? That cost money at most airline terminals. As for WiFi on the plane, that also cost money (not that you can use 3G up there either though).
I will find 3G useful when I'm in the car going on road trips. I will find the GPS useful as a lot of programs use that.
My main reasoning is that I don't want to miss out on any apps and having both 3G and GPS make it worth it to me. I figure, $65 for the 3G and $65 for the GPS ;-).
All my cars have factory Nav systems except for an S2000 which I have a Garmin unit for. All of these are much better at navigation than an iPad.