> User-settable keys also compromise the security of the device.
Not if a physical modification to the hardware, a set of button presses, a hardware token or a fuse is needed to set said keys.
> And in general it's risky to introduce stuff like this because the potential for unforeseen security compromises is very high
There is little to no risk if physical interaction with the hardware is required for setting keys. If you have physical access and enough time, all bets are off anyway.
> ...and the upside is very low (the number of people who'd actually take advantage of this is extremely low relative to the number of iPhone users).
There are many people who dream of having the freedom to roll their own firmware on a particular device and it can be done without compromising the security of said device. This was not just about iThing users (who might/not take advantage of this), this was about people spreading FUD about the tivoization part of the GPLv3. Root signing keys don't need to be shared and the only requirement is that the user can take the sources, compile them and then run the result on the target device. As long as the OEM distributing the GPLv3'ed program gives a clear path for running the compiled code on the target device, then it is not a problem.