I think that if used ‘properly’ they can be a good tool. In my opinion, whatever tool speaks to a particular person and gets them motivated to engage in HEALTHY exercise is potentially useful. Since so many in our society are focused on and motivated by the cosmetic aspects of fitness, ‘before and after’ pictures can be useful. I would caution however that this ‘motivation’ needs to be tempered by the person’s trainer so that they are not solely, or even primarily, motivated by what they see in the mirror and use that as their sole or primary indicia of becoming healthier.
My two cents.