Earlier quoted context omitted.
I can understand why people use 60/70% keyboards. I personally couldn't give up the numpad, home/end/pgup/pgdown/arrow keys for it, but there's definitely something to be said for the comfort level of having the mouse less far off from the keyboard. I think once you're into anything less than 60%, either you literally just type text and use the mouse/touchpad items instead of keyboard shortcuts, or you're more focuse…
I've had my eye on a few 40% builds as of late, so I'll throw another option into your list here: portability. I'm looking for a keyboard that I can throw in my laptop bag that isn't miserable to type on. I had a Microsoft Designer Keyboard for a while but it's worse than my laptop keyboard, so I'm back to the drawing board, and this seems like where I'm landing.
Maybe if the bag was filled for plane travel that'd be a problem.