Never happened to me yet...
I think that if you take care of your tip you shouldn't have a problem or frear that you could "lose" it... When it's time to change it do it to a reputable repairman and you are good to go. I don't think that you should spend $200-300 on an extra shaft that you have realistic chances of never playing with, because nothing went wrong with your tip...
I can understand it tho maybe if you play big money games, and you wan't to be 1000000% sure you are covered...
It happened to me. I was wiping my cue down to put away my cue for the night in between matches of a tournament and I popped the damn tip off!! This was 2 AM, I had until 9 AM in the morning to glue it back on. It turned out to be a non-issue but if it was between matches during the day it could have been unneeded stress. Ever since I have been meaning to get a spare shaft, I will follow thru some day. :grin-square: