There are no absolute reasons for carrying more than one shaft. You need to decide what you why you would want to carry multiple shafts.
Years ago, I was taught to carry two identical shafts with identical tips and play with them on alternate sessions so their condition would progress in parallel. That way if a tip falls off during play, you can switch and be comfortable with the other shaft. In 45 years, I have never had a tip fall off after more than a week of play. Only two tips came loose in less than 7 sessions.
Three top tier cuemakers have told me that many of their customers have shafts built to different specs possibly with different brand/hardness tips to meet their preferences for different styles of games, e.g., 1-pocket vs. 9-ball.
I carry two playing cues. I am sufficiently protective of my favorite cue that I use a separate cue for league play where there is too much commotion. There is no good reason for carrying second shafts for each of those cues except that the case has slots for them.