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.
Beside moving the cursor in a textbox it's also nice as blinking orb of information. For mine if it's blinking blue I have a tweet waiting and if it's purple that's a google voice message, etc. And it blinks red and then blue and then green I have three things waiting. Armed with that kind of information I can make a decision about if it worth getting the "stare of death" from my wife if I pick it up at dinnertime.
What I really need is to have it blinking red if a new Angry Birds is available at the market.
I wager that it was a way to attract Blackberry users.
A lot of devices (like most of the Galaxy S family) actually don't have a trackball/pad at all now.
I very rarely used the trackball on my old HTC Magic, and haven't missed its absence at all on my Galaxy S. It's not needed these days, the touchscreen is enough.
I suppose to best "answer" your question, the trackball is there to assist when the touch screen isn't accurate enough. It also exists to give people more choice when scrolling and or selecting.
Personally, I never use it for that but instead use it for Trackball Wake. Not having to push the power button every time to turn the screen on is a god-send. I, like others here, also run LED customizations to notify me of specific things.