I'm sure many here have worked with keyboard players. How many times have you seen them bring a redundant board? Just asking??
Keys are different. While it's great to play exact sounds for cover tunes, the audience is rarely upset by piano and organ renditions in a band context. I always bring more than one that can cover piano and organ just in case. You can't prevent things coming apart internally on a keybed or inside the case. Sh*&#%t happens!
I'm a little strange when it comes to being prepared but I wouldn't consider someone unprofessional just because they came with one axe, but when someone shows up with a spare I make a mental note because it gives the impression that they may be more professional than many others.
I always have a backup bass or guitar, and I carry a Tone Hammer as a backup pre/direct box plus a tuner in case my amp fries. Also bring a backup cable, strap, speakon jumper, 1/4" jumper, set of strings, leatherman, butane soldering iron, multi-meter, cable checker, adapters, duct tape, etc. My little briefcase carries a lot of stuff. Chock it up to to many years mixing sound, and maybe augmented by a little paranoia being lefty 'cause I can't borrow an axe to cover.
I've been able to lend a drummer a drum key, tools and neck stand to guitarists, and even a strap to a guitarist more than once. I've probably sold 100+ 9-v batteries for $2US each over the years. So I carry that briefcase to most gigs even though I almost never have to open it.
Mark