Yeah, I could easily envision an attachment that has no analog stick just 4 face buttons for the thumb along with the triggers.
As it is, I think the 4 dpad directions, the A button, the B trigger and the 2 (?) triggers on analog attachment are the full complement of buttons that would be accessible to an FPS game.
What I don't think I have a full handle on is the kinds of motion that the wand detects. I think it can detect rolling motions, pulling/pushing motions in addition to motions up, down, left and right.
I can imagine rolling motions being used to peek around a corner for example. Pushing towards or pulling away could be gestures that initiate crouching or standing. Up, down, left and right would obviously allow you to look around and the analog attachment would control movement.
The point of my bit of speculation is that there will be different ways to do things so I'm not concerned about the number of buttons yet. The buttons just complement the central wand functionality. They allow the same motion to mean something different depending on the button state. Like with a mouse, you simply move the cursor if the left button is not pressed, you drag stuff if it's pressed, you summon a context menu if the right button is down, etc.