jobo
Grandmaster
the lower back should be very very strong, think dead lifting 400lbsThere are better exercises for developing the lower back than a bent row. I'd advocate using those if lower back strength is low. But that's no reason to stop developing the upper back, which is not entirely useless just because it is more developed than the lower back. It won't provide its best utility unless/until they get that lower back better, but unless they create a deep imbalance (unlikely if they are directly targeting the lower back with other exercises), it won't be a problem.
if the exercise as suggested to to much, you don't have a weak back you have a very very weak back. Developing your body of out functional balance is very dangerous and one of those old fashioned training methodologies' you hang on to