The evidence suggests that the set you get put into for any of what used to be called the 'core' subjects (i.e. English, maths and science) is the set you'll stay in for the rest of your schooling. And that's before you control for things like gender and socio-economic class, when the effect worsens.

This, in itself, is sufficient justification to get rid of streaming /until a way can be found to do properly and consistently across the education system/.