For the type of application not only Google Earth.

Because double tap allows you to conveniently zoom in on the place you want to read/see. When that doesn't work well, such as in Google Earth, you need 'pinch' gesture.

Yes, I can.

To clarify: let the device lay down on your hand palm and spread your 3 fingers. Then use thumb and point finger to zoom. After that you can use either one of those fingers to to move left/right/up/down.

Case in point: I've used this in Munich, in the rain, while using my iPod touch to load some Google Maps from an open WiFi AP.

Its not what I'd call convenient but usable for emergency or quick usage. For most people however something, e.g. a seperate hand, has to hold the device.

I can do what you imply with my E71 (I zoom in, out, and move left/right/up/down) easier than I was able to do that on my Nokia N810 however it still takes various buttons to press and therefore attention/focus. Multi touch gets me to the map quicker because I visually 'travel' through the map. With buttons it feels more ar-ti-fi-cial.