Yeah yeah- develops imagination, awareness of design, materials, planning blah blah blah. But I don't want to struggle home with a robot shedding milk bottle tops, I don't want to tell my child that dh has 'accidentally' recycled their lighthouse and I don't want my child digging through the bag of recycling and distributing random bits of refuse all over my house. There I've said it, and actually I don't mind if I am BU, I still hate it.