It seems to me that there are (at least) three different goals that different people might have in mind for demo/eval/development kits/boards

Please note that I'm using the terms purely for the sake of argument - I am not intending to assign any specific meanings here.

Demonstration
The board is really just to show-off, "look! it can do this!"
In this case, it doesn't matter (much) about source code or hardware access - you're just supposed to see it doing the demo and say, "so it can!"

Evaluation
The board gives you a bunch of common/"useful" peripherals to "play with" in order to get a "feel" for the device.
Source code here should meet the full exemplary standards, but the hardware is intended to be self-contained rather than for adding your own.

Development
This is to get you started on a real design; ie, to do some real development on.
Again, software should be high-quality, and there should also be easy access for adding your own hardware