And, good sir..I would humbly tell you that there are NUMEROUS benefits to your analogy of a 45 yr old woman doing seated presses on a balance ball. A few being: core strength, improved pelvic and spinal posturing, as well as the stabilizer muscles we discussed..
I disagree. I think it's baloney. How does being unstable build core strength?
Not sure what you mean by "pelvic and spinal posturing."
And again...what exactly are the stabilizer muscles to which you refer? All muscles stabilize.
Train for strength, shape and size. That means heavy weight while solidly positioned. For posture and balance you can do yoga type stretches. Lifting weights while standing on one leg is not the answer IMO. One or the other.