Well that is the difference between a development board and a demo/evaluation board.
The development board should be minimal on its peripherals to allow for easily hooking whatever everyone needs to work on his code.
The evaluation boards should have most the peripherals that they can fit on its board, so that users can try how the environment works with them.
But that is completely different than having a small button hooked up on a pin.
I think that a general purpose button on lets say pin 12 would be very useful for fast prototyping and really easy to implement on the same pcbs. In fact, it can be easily designed to be easily ignored by making the pin available on the headers.
A normally off button is electrically neutral when not pressed, so if you don t need it, you can always hook whatever you want on its header outlet and just ignore it.
We can always do it on our own anyway, but we could do it with the led as well, even without having to sacrifice pin 13 for good.
Plus with the button, we won t be sacrificing anything besides some space on the pcbs, as I explained above.