In the 9 years where I shot without a backup gun, I could have used a backup only twice - when a trigger started to double. Instead of getting a backup gun, I got a backup trigger group (full hammer, sear disconnector set) but never had to use it after that.
During the last 3 years, I've had a backup gun, and had to go to it once. The night before a match, I found my barrel link was cracked and I was not able to get it to a gunsmith to replace the link for a few days. At a match, somebody else's gun died on them, and they were able to use my backup gun.
Other than that, my backup gun has gone unfired in the last 3 years.
I probably wouldn't buy a backup gun unless you're buying a new gun and relegating your old one to backup status.