I carried one for many years...15+...and never needed it while out hunting. For that matter I have never had a string break on me while shooting for 20+ years of traditional. Maybe it was luck...maybe it is because I use generous amounts of string wax on them...I dont know. I dont carry one now unless I have my haversack but only for that particular bow I am using that season. If I grab a different one for some reason...I will be SOL as I dont want to carry 10 different strings for different types of bow, string materials, weights, etc.
It is a good idea but I guess it is like a spare tire or a handgun.....it is a tool....and you hope you never need to use it but are thankful it is there if you ever have the need. So far I have not had the need...knock on wood.