As far as I can see, if you hold a controller 'as normal', using the right stick for aiming, you have simultaneous access to L1 and L2 with your right hand.
If you use the touchpad instead, to use it comfortably, you would lose access to L2 and R2 but you gain access to the pad's click. Which would be enough for point and shoot.
Meanwhile your left hand can be doing all the same things with the left side of the controller that it always would (i.e. character strafing, forward, backwards etc.)
I remember when rumours about Vita's rear touch came out, I decided to try some FPS games on my laptop trackpad. I was much more competent than I am with an analog stick for aiming. Particularly for very subtle, fine-grained changes to aim. With a fixed trackpad size for games to work around I think it could be very interesting. (Of course, in Vita's case it turned out that it was very difficult to use the reartouch while keeping simultaneous access to a button, for firing or whatever. But PS4's pad doesn't have that problem).