Is the person training for performance or vanity? It all depends.
I use stabiblity balls, balance equipment, resistance bands, and med. balls for training, but those are supplementary, mostly for specific sports conditioning and overall flexibility and balance. For building muscle, stick with traditional exersises.
I've been to a few seminars of Juan Carlos Santana and he touches on this a lot. It really opens your eyes, especially if you're a Personal Trainer.
