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The jump cue is a part of our game because of marketing and retailing. It's fun to watch, but bad for the game (from a purist's perspective). What's wrong with jumping with your own cue? Ahhh that's right--- most people can't. It's like using the "Alien Wedge" out of a bunker on a golf course. My mom can blast out of a bunker with one and she doesn't even play
Sorry I didn't address this part of your post.
First, the jump cue was invented by a world class player and that player never sold or marketed a jump cue in his life.
Jump cues were invented and refined to fill a specific need. And that need is to not give up ball in hand. Since the rules changed to one foul instead of push-out it made the game lopsided. When a person would deliberately play safe or get safe through luck then the incoming player was faced with a really tough situation with a really harsh penalty, ball in hand for the incoming player.
You see the people who wrote the push-out rules for nine ball were not stupid. They understood that getting lucky to safe a person and thus giving up ball in hand was not a pleasurable way to play the game. So they made the rule to eliminate lucky safes by allowing the incoming player to push the cue ball to another position instead of forcing them to try and hit the ball. This rule worked very well BUT it could lead to some long games.
So the guys at who formed the Texas Express tour did so with their new Texas Express rules. In these rules there was no more push out at any time allowed, only after the break. Thus the game changed in a critical way so that now lucky and intentional safeties were much much more powerful. Until that time very few people had good kicking games or good jumping skills. Those that did often exploited them under the old rules as they would push to a kick or a jump knowing that the incoming player would give the shot back.
in any event the new rules changed the game making it so that the incoming player MUST contact the lowest numbered ball and a rail or give up ball in hand. And this is why jump cues were invented and introduced. In order to somewhat reset the balance between defense and offense in the game of nine ball.
Many people have worked these past twenty years to refine the jump cue into a real good tool that fits the game. Rule makers have refined the rules to define the parameters of cues so that jump cues are made within those parameters.
There is no consortium of jump cue makers out there lobbying the rule makers to allow their products. It's simply a natural evolution of the game born out of the way the rules were changed and how the game itself changed along with those rules.
Now to address the second part of your statement;
"What's wrong with using your own cue to jump?"
Nothing except that all "normal sized" cues are not equally suited to the task. Some really do "jump" better than others. So if I happen to own one which "jumps" great and I am playing against you and your cue doesn't jump great then I have an advantage over you that has nothing at all to do with my skill level. If we switch cues then the advantage goes to you when it comes to jump shots.
And the other disadvantage to the player is that jumping with a full cue is very awkward. The cue was not designed to be used that way so the player has to contort quite a bit and it's frankly physically challenging. The "normal" pool cue has evolved into it's present form because that is the optimal form for playing pool with a level stroke. Going back in time you will find that there have been many variations of the pool cue.
So a jump cue gives all players the same tool and the same range of shots.
How good a player is using that tool depends entirely on their own skill.
