The problem isn’t with the students or the material, the problem with streaming lies in the very structure and nature of the classroom itself. If one follows Vygotsky’s conclusions to a logical end, EVERY student is on a different level, and so there is no happy medium for any student, high or low, except the point whichever single student and the teacher and the material and the method conveniently meet. Every other student past that point receives increasingly less effective instruction.
If you have 20 or 30 kids in a room, receiving instruction from a single teacher, in a single subject, at a single moment in developmental time, and every student is unique, then inclusive instruction forces the entire system away from exclusivity of content, method, assessment, instruction, and so on.

Having other students teach material to their peers is only one of the many benefits of an inclusive learning environment, but in my experience all of them are fundamentally productive because they push against the counterproductive nature of the traditional classroom environment.