I guess it comes down to how many people find keyboard brightness to be worthy of a keyboard assignment.
Even if I had a lighted keyboard (since I don't have a MBA/MBP, it's not a thing), it wouldn't matter much to me. I rarely use my Macs in dim/dark light and as a touch typist I rarely look at the keyboard at all. The chances I'd be disappointed by the default brightness setting seem remote to me.
On my Apple Magic Keyboard there's no default assigned to either F5 or F6. I can't say I'd find dictation or DND useful additions either. But for that matter I rarely use most of the F row - most of the time it's to un-do an accidental press of an F row key, so if the F row had no default assignments at all I'd probably be ahead of the game.