Having a joint protector on is just one those "I'd rather have a gun and not use then to need a gun and not have" thing.. If by any chance I drop my cue pin down, chances of damaging the joint would be lessened I guess.
That said, I would say protection is only 50% of the reason(for me)...the other 50 would be for aesthetic reasons...the cues just look sweeter with nice matching JPs...
From the thread I would assume the OP is more into the game than cues? So you're probably into Sneakies, plain janes...basically straight forward players? 
Well for the same reason I like my cues with nice inlays be it ivory, silver, gold or unicorn's horn they don't make my cue(or me) play better...they just make my cue look sweeter! Same reason I like my cues with 268 Recuts and not just 4 simple pointers. Same reason I use elephant ear wrap on my cue as oppose to cheap lousy linen. They don't make my cue play better at all...
Also the same for cases...Ron Ross tooling on my case doesn't protect my cues any better. Using python, alligator or elephant hide still doesn't offer more in terms of protection..
To sum up my long winded post, there's still an element of protection but to many there's the aesthetic factor..