Never taken a spare guitar or amp. Main reason being, I've only ever really owned one at a time
One guy I knew always took a spare bass and amp to gigs, but by the time he needed to use them, they were crackling and squealing and cutting-out. He'd gone so long without using them, they needed a damn good service!
It might be tempting fate not having a spare, but you definitely reduce the chances of something going wrong if you use decent cables and look after your gear.