It all depends on your goal with training. If you are training to sell, I agree. If you are training your team only and are not concerned with the TL or falling behind the Joneses, three position training is ok. I frequently train team or three positions, but I am aware that players improve even if they don't get a little green arrow.
In general and for most people, I think you are probably right. I am training OD 3 positions as we speak, though, so I can't agree for myself.
Once I scored a basket that still makes me laugh.