I find this question very interesting as well. The tangent discussion on usefulness and time wasted aside, how this item first came into being and why it continued and evolve to present day may be small, but it is interesting.
The oldest so far is just a cork. It may not stop a bent pin, but it will reduce the chances and definitely will protect the threads from damage. The slipcover over the ivory joint also makes practical sense. I don't think it can be denied that joint protectors give added protection to the pin and its threads as well as keeping debris out of the shaft and protecting its face. Whether a person thinks the extra protection is simply personal and does not differ from the endless debate over JB case's added protection over other cases. (go to the Case forum if you want to continue that debate)
Enough people over the years felt the joint protectors were important so they eventually became a thing. Of course, there is some accessory marketing added in. Like cues some people are happy with a plain jane and others like the extra pizazz or custom cue. Joint protectors also come in plane jane Delrin to fancy or even custom JP's matching your cue.
Time wasting is a bit of a nit as an excuse not to use them. If you want the benefit they provide, the extra few seconds mean nothing. Why waste all that time putting on a glove, you could be on the table already? I don't bother with them because they don't benefit me, but I do acknowledge the benefit they provide.
Hope someone provides more info on this Scooby Doo Mystery.
Obviously, I am on the approval side. I have always used them. First the delrin ones and now that I can make them custom, I just like the extra "look" of them with the cue.
Here are a few favs.
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Made to match an Ariel Carmeli cue
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Spare shafts
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Made to match the cue
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Custom for cue
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Cue and JPs in progress