Your win percentage against the 9Ball ghost will, over time, show if you are improving in your shot making and position play. The one problem in playing the ghost for practice or training is that there is no safety game inherent. You have to go for every shot no matter how difficult because all misses are losses. So it is weak in terms of training for defense and safety play, and because of this it is also weak in playing proper strategy. So you don't want to get caught up in just playing the ghost to train.
Having said that, there are ways you can incorporate safety play and defensive strategy in playing the ghost. For instance, if you are faced with a difficult shot or table layout, you can choose to play a safety. If you hook the ghost (i.e. has no direct shot at the object ball and would have to kick or jump), you can call the game a draw. Or you can shoot for the ghost and give it your best effort, then play the game from there. There are other alternatives you can create.
I like these variations exactly because they bring defensive play and strategy to my practice.
Just keep in mind however, that around here and elsewhere, when referring to playing the ghost, it does not include these alternatives. So, if you post on here that you beat the 9Ball ghost 70% of the time, people are assuming you mean in it's pure form, without safeties etc. (you miss - you lose). But for your own private practice, do whatever floats your boat.