looks like we will have to take away our eyes from the screen only to position our thumbs at the center again and again. Like in touch screen devices. Analog sticks resume to their original position if you let them. You don't have to look where you touch. That's why analog sticks are good.
There are ridges in the trackpad indicating their center, and at another radius further out from the center. So you should be able to position your thumb in the center without looking down.
In a game with a fixed cursor (like FPS) this is also less of an issue.