I mean, in the absence of evidence, what does the phrase "good in and of itself" mean other than "I have prejudices"?
Well, if by "prejudicies" you mean "principled political positions" then yeah, you're right. Though "prejudices" is quite a loaded way of describing them. What I mean is, if you think exposing your children to a mixed-sex environment is a good thing for them, then the "evidence" can't be anything other than, well, that they were exposed to a mixed-sex environment.
Until "getting on well with a range of people" can be deduced scientifically, I'm not sure where you can go with it.
Education debates are inevitably a battle of ideas - or "prejudices" - I'm not sure who actually is loftily neutral about it really.