Sorry to butt in here.... 
But the pro-grammar school crowd have to ask themselves questions:
-When they say 'GS' do they mean 'super-selective, 2% schools' or do they mean '23% schools'?
The first I can agree with: such DC are often so 'marching to the beat of a different drum' that they would benefit form highly specialised, academic teaching maybe not available in most schools. The 3-23% - well, no. That concept really does smack of 'keeping my DC away from the Others' even though they really aren't that much more 'able' than many.
Do the pro-GS crowd agree that they would readily send their DC who failed the 11+ to 'the local SM' (though many like to suddenly call them 'comps' as it sounds better and is only really valid if only maybe the best 2% have been scooped off, but the top 23% gone? No, you have a SM). OR in the interests of 'only 'what's best for my DC' which, on MN, appears to ride roughshod over any concepts of 'the common good', they'd go private instead.
Do the pro crowd accept that IF they view a GS education as delivering the 'most suitable' education to the able DC (by which most seems to actually mean 'my own'...), a damn sight more cash will need to be spent on providing a 'suitable education' for the less academically able? Which might mean 2 or 3 different types of school, and which would definitely mean considerably smaller class sizes, more innovative and versatile teachers as the 'less able' require rather more input that the clever (please note, as above, that the private spoken about has classes of 25 and even Xenia tells us her DDs amazingly, incredible hyper competitive north London girls private schools have classes of 26 because the DC are a) so able they need no personalised tuition, b) that they're all of the same, very high intelligence and c) there are absolutely no discipline issues...) AND bear in mind you do not necessarily get the 'best' teachers in a 23% GS as in 'those most able to inspire the widest variety of DC'.
Do the pro-crowd accept that all this costs money that just might be taken from GS pot? Or do you think that in reality, our class driven society will conspire a way to ensure that 'the best' (and I haven't said best academically) is only available to the sharp elbowed and wealthy? which, incidentally, is what someone on this thread has accused the Tories of in closing grammars in the first place- a ploy to remove a tier of competition challenging their own privately educated DCs 'rights' to wealth and influence.