What's the purpose of the trackball? what's the point of the trackball of the Nexus One and of the analogous devices in other similar phones? (e.g. HTC Desire's optical pointing device).

Is there any use case for which the touchscreen is not enough?
I find moving the cursor to a particular place in some text far more accurate using the trackball than trying to select it by prodding the touchscreen. I would say that there might not be a specific use case which can't be accomplished using only the touchscreen but that there are certainly tasks for which using the trackball is easier.

It's all probably quite subjective though, and depends on user preference.