Here are a few I have culled from the plethora of threads over the last few days.
Learning to fail is an important part of learning to win. Secondary modern school children learn this valuable lesson by failing the 11+. Grammar school children learn it by watching the secondary modern school children fail the 11+.
There is no bullying in Grammar schools.
Grammar schools are full of gifted academics.
It is good for a middle ability child to be the top of their class- they find this empowering and motivating. It is bad for a high ability child to be top of their class, they find this boring and demotivating.
An A from a grammar school is better than an A from a comprehensive or secondary modern.
Children who fail the 11+ can get jobs in music, drama, art, sport or "working with people"
And, of course, the classic "Why on earth should a 10 year old who fails the 11+ feel any sense of failure?" 
There are plenty more- feel free to add your favorites.