I have one of these too (including the terrible logitech portrait) and I think the important verification going on is that 1) once you're approved, your passport number is linked and the book can be cross-referenced and 2) I believe DHS is now doing facial recognition at global entry terminals, which does away with the card totally.
Oh, and my picture on it is laughably bad. I distinctly remember the border agent using a consumer Logitech webcam to take it.
[1]: https://en.wikipedia.org/wiki/Global_Entry