1. Billiards has always been a 2-dimensional game. I feel there is something inherently wrong with deliberately leaving the boundaries and manipulating the physics of the playing surface with a special piece of equipment.
2. It takes the skill out of the game. People argue that you need skill to use a jump cue, but it is minimal compared to the amount of mastery required for one to execute complicated kick shots.
3. You can still jump with a full cue. If you can't hit a home run, you should work on your swing and mechanics rather than investing in a corked bat. The jump cue is just another piece of equipment we need to buy just to stay on a level playing field.