Why do Arduino shield kit designers "consume" pins they don't use?
I suspect that most shield designers either don't think of it or run out of time. When I've made shields (for personal projects), I've had intentions at the start of adding other features, but by the time everything is laid out, I just think, "OK, this will work. Maybe I'll add that other stuff on the next rev."

Adding blank holes for hacking on extra stuff is a great idea. Given that most (all?) of the shields on Sparkfun are open hardware, why don't you make an improved version? I suspect that the Sparkfun folks would be delighted to accept your improvements.