I think you simply dont understand the difference between being able to jump and being able to jump well. Buying a jump cue only allows you to make a jump easier, just like a break cue will allow you to break better. But to break or jump well, either making the balls or playing a counter safety with a jump shot, requires skills, and that takes you lots of time and effort.
No one can pick up a break cue and start breaking with tons of power, with precision and control at the same time, similarly no one can pick up a jump cue and start jumping and making the balls all the time.
Jump cues just make jumping easier, just like cues makes playing easier, it is up to an individual to hone his skills in order to excel in that particular set of skill.