"If they just want to learn, they will have been applying themselves in primary school and will pass the 11+. I don't see why the bright x % should be penalised"
No - there will be very intellectually bright children who may be very weak in one area of the curriculum who will fail the 11+.
There will be bright children who are late developers who will fail the 11+
There will be extremely clever children who do not sit the 11+
There will be very bright children who have undiagnosed ADHD or ASD which has impacted hugely on their attainment.
There will be extremely bright children whose attainment at 11 is way below what they are capable of because they have had no help or support with their learning at home, sometimes because of a lack of interest from parents sometimes because of long term illness in the family or severe poverty. My middle dc's learning has been hugely affected by having one sibling with ASD and one with severe mental health problems - I simply have no had the energy or time to support his learning as much as I could have.
And then of course there's the fact that 1 in 4 children are tutored. And a hugely disproportionate number of grammar school applicants are coming from private schools.
The point being: YOU CANNOT ACCURATELY IDENTIFY AT 11 WHICH CHILDREN WILL SUCCEED ACADEMICALLY given good teaching and support, and it's wrong to base an entire education policy on the view that you can.
All state schools need to cater for children of all abilities because they will ALL have children in them from across the ability range.