If you have a custom, I firmly believe you 'need' 2 shafts for it, especially if you play money sessions or big tournaments. If I had a custom worth over $2,500, I would have 3. I don't do all the different mm size tips or different tips on playing shafts.
One area that is overlooked, in my opinion, is the need to have 2 shafts for your Break cue. What if you were in a big tournament, and a tip came off your break shaft, or worse yet, your break shaft broke. How much would it affect your game to have to use a house cue to break, or your player cue to break?
Considering the break is one of the very most important shots in Pool, don't you think have 2 shafts for a break cue is a good idea?