There are very good reasons to carry two. I keep two as close to identical as possible. When a tip begins getting mushy, which happens, I will swap out. When I ding my shaft & the dent is bugging me, distracting me, I'll swap out. If it's humid and the tables are dirty, the towel just isn't doing the trick anymore, I swap out. These are all typical, every day reasons to have two shafts. Beyond this, it's nice having a spare when you have a tip blow out, pop off, ferrule crack, etc. I can't count all the times I have seen a cue get knocked over & bang everything on its way down, leaving it feeling more like a dildo than a cue shaft. No reason to quit the day, just swap out.
Now THATS funny right there! Disconcerting.....But funny!