I still wouldn't call that a waste though anymore than I'd call model building a waste. They may not be using their imagination but that does not mean they aren't learning or developing other skills.
They are learning to follow instructions. Sure that's a useful skill, but 98% of school is teaching them that. What the more open ended lego kits teach is that you are free to create things constrained only by your imagination and the bricks you have. That surely is far more valuable than reconstructing a movie scene and having it gather dust on a shelf.
