Was about to comment on this on another thread, and decided it was too tangential to the issue, so am starting this.
One of the things I often see on here is 'comprehensives are fine as long as there is rigorous setting - and I must say I've always thought that's how I feel too. DD was initially set only for Maths and MFL, the rest came in year 8. But now in year 10 they aren't set for optional subjects - in her case, French, German, History and Geography. So still set for English, Maths and Science - not actually sure about RE.
It came up last night when she mentioned that today she is invited to some 'student summit' or Big Conversation type thing where they get to say what they think is good or bad about the school. The main thing she wants to say, btw, is that it's not fair that year 9 can go in any toilets, but year 10 can only use the year 10 and 11 toilets 
So I, being a bit helicoptery don't-want-to-put-words-in-your-mouth-but... said 'yeah, and the sets in MFL and humanities, they should set you again!'. And she just looked quite surprised and said 'nah, I mean my French group isn't great, but I'm doing alright, aren't I? I think it's ok!'.
So I was thinking - top sets are presumably nice for the pupils in them and the teachers who teach them, but perhaps not having them isn't the disaster I and many others often think? I was really surprised that she felt that way, and it's made me think about other things, like children being in the 'wrong' set, or the top set getting to do things the others don't....
Do we need to worry if they're not set? Is setting actually more about the assumption that the well-behaved kids will be the brightest? I wonder.