I am probably wrong, but I think I disagree 
I had a set of joint protectors included when I picked up my mcdermott. I asked what the purpose was and they said everything that has been mentioned here....keep crap out of the joint, protect the pin and collar, blah blah blah. I really didn't see the purpose, but I figured, who cares, they are free and I care about my (little) investment.
A buddy of mine bought a similar cue 3/8x10 wood/wood joint. He never used joint protectors and his cue got harder and harder to assemble and dissassemble to the point where it would take two (weak) people to break his cue down every night.
I took it into a shop and we had to tap the joint. For those unfamiliar with this, we took a really sharp screw 3/8x10 and re-drilled the shaft because there was allegedly so much crap in the shaft joint that the cue was nearly impossible to put together or take apart.
Needless to say, I picked up a cheap set of JP for him and he has had no problems since.
Are they worth it? my limited experience says at least with McDermott wood/wood joints they are.

I had a set of joint protectors included when I picked up my mcdermott. I asked what the purpose was and they said everything that has been mentioned here....keep crap out of the joint, protect the pin and collar, blah blah blah. I really didn't see the purpose, but I figured, who cares, they are free and I care about my (little) investment.
A buddy of mine bought a similar cue 3/8x10 wood/wood joint. He never used joint protectors and his cue got harder and harder to assemble and dissassemble to the point where it would take two (weak) people to break his cue down every night.
I took it into a shop and we had to tap the joint. For those unfamiliar with this, we took a really sharp screw 3/8x10 and re-drilled the shaft because there was allegedly so much crap in the shaft joint that the cue was nearly impossible to put together or take apart.
Needless to say, I picked up a cheap set of JP for him and he has had no problems since.
Are they worth it? my limited experience says at least with McDermott wood/wood joints they are.