Also CON: it could confuse anyone who even understood the key ever, but PRO: it could help teach people how to disable the annoying CapsLock.
The current Apple keyboard (i.e., the aluminum one) doesn't send a key down event to the computer on a tap to turn on caps lock. You have to press and hold. It does send a key down event on a tap to turn off caps lock. That is, it's biassed against turning on caps lock, and towards turning it off. I think this is a terrific feature. I hope it catches on. Its antecedents are manual and electric typewriters where caps lock had precisely the same behavior (albeit implemented mechanically). However, this makes the caps lock key a bad choice to map to a modifier on an Apple keyboard. Half the time, the keyboard won't send a key down for caps lock and you won't get the modified key. (Yes, the hardware itself does this.) Making this the default, besides being unintuitive, will seriously annoy anyone using the current Apple keyboard.
Do we have to remap only one key? What if we allow Windows _and_ Alt keys to trigger our keybindings by default, as if there was no difference between the keys, to xmonad?
This doesn't solve the problem being posed. People change the modifier to the Windows key because they don't want the xmonad to use Alt key combination. (e.g., I use the Windows key for xmonad because I use the Alt key for emacs.) john