Seeker - I personally believe in selective education.
Just as girls tend to do better when taught in single-sex schools, I think all children do better when taught within a similar ability group.
The issue in the past has always been that more time, effort, money and other resources were invested in the grammars than in the SMs which was very wrong.
In very good comprehensives they manage to achieve the teaching by ability groups, but this is far from the norm.
It allows the very able to work at a faster pace and on the whole disruptive elements are eliminated, it gives average students the chance to shine and also not to be ignored in the way they often can be in a mixed ability class where a teacher is trying to stretch the able while help the struggling. Finally, it means that those at the lowest end who are really struggling can have dedicated help, and perhaps not feel they are failing compared with many of their classmates.
Grammar schools are obviously popular with a lot of parents - judging from the numbers sitting the exams and the tutoring industry so it makes sense to expand to meet some of this demand.
I'm probably also biased since I went to a grammar, and I live in a part of London with dire comprehensives. I imagine in smaller towns that a comprehensive can draw from a wide catchment in terms of socio-economic background and educational ability/attainment.
In my area, the catchment is so small due to population density that there is no wide range at all - and frankly less than 50% getting 5 A-C grades (and no-one getting a spread of A grades) doesn't make me feel it's somewhere I want to send my daughter.
I get the choice of moving house, raising the money for an indy or trying for a grammar school somewhere. Until everyone has the choice of a really good comprehensive option then grammars will be the lone hope for those without the funds to move/pay privately.