Unfortunately, many trainers don't know the difference between making people tired and making people better. They think that if a workout exhausts people or makes them sore, then it's good. When you do circuits with 10 or more random exercises, you certainly get folks tired, but the excessive variety prevents people from adapting to the exercises. The effect is too general for adaptation to occur.
We used to do this too and what we found was that after 6 months, people stopped progressing. They stopped progressing because they never got stronger at any movement. How could they when they were doing 10-15 moves? We switched to a combined strength/conditioning model and our folks haven't stopped progressing. The skies the limit.