Ah! Yes, that would make sense, and might actually work pretty decent, as long as you get the sensitivity in-game right. Could be weird to aim in an FPS if you ever need to take the thumb off the pad though, hopefully there's a sufficient (but also not overexaggerated) deadzone in the center so your view doesn't suddenly snap aside slightly because you put the tip of your thumb down again.
I think you might be confused... but it's probably me!
Putting your thumb down anywhere on the pad is never going to cause the view to "snap" anywhere. It'll be just like a mouse. You move it in any direction until you run out of room. Then you pick up the mouse (thumb) and set it back down
anywhere you want on the pad. Then you can start moving it again. Just like with a mouse, as long as your thumb isn't moving across the pad as it touches down, you view direction shouldn't change much at all. We've certainly never needed a "dead zone" in the middle of our mouse pads!
Joysticks and touch pads can be quite different in operation. Joysticks are typically "indirect" in operation. Tilting a stick and holding it there results in continuous motion in a direction. That's indirect. You're requesting motion even when your thumb is motionless. Sliding a ways on a touchpad and then stopping results in the motion stopping as soon as the sliding does. There's no need to return to center in order to stop the motion. You just stop moving your thumb. The motion is mapped "directly".
But... indirect motion can be good too. In an FPS your body motion is typically controlled either by a keyboard (WASD), or by the left stick of a controller. In both cases, the control is "indirect". If you "push" in a direction, you will continue to move in that direction until the key (or stick) is released. And that's fine. Direct motion mapping would suck for moving through space. (Imagine moving your character forward by pushing a mouse forward. Over and over again. Wolfenstein-style.)
So, the Valve controller will need to allow us to map either pad to either a direct (mouse) style control scheme, or an indirect (stick) style scheme. (The further from center you slide your thumb, the faster you travel.) For an FPS, many people will want indirect on the left, and direct on the right. Analog stick emulation on the left, mouse emulation on the right.
It's a brave new world, folks. Hold on to your butts.