I think they just use the light so they know which object in the room actually is a controller
Of course it's a nice bonus that it works in the dark too. But I'm pretty sure it just tracks the 2D position (up/down, left/right movements). That's all they need for basic UI control, split screen detection, sixaxis movements etc.
When they use 3D tracking it basically becomes a less accurate Move controller. Which seems like a stupid thing to do, because then devs have to program for both an accurate device and a not so accurate device. Of course they could only support the Move controller for their game, but then 3D tracking becomes completely pointless in the DS4. I think technically it should be possible with the camera, but I don't think they are going to implement it.