I think there was quite a heated debate on this about a year ago when I was going through the whole school decision !
Personally I think its great marketing! If you get "selected" you believe that your have made it into an "elite" school. Whilst I really do not dispute that this is an incredibly good school with amazing facilities I really don't think it produces better results than the other local schools (private and State!) Interestingly when we were deciding whether to take the place we had I could not find any details of their academic results. Most other schools in the areas have these on their websites - this could have changed now !
I personally just disagree with Assessments at that age. Kids at 3 can easily have an off day. Also I have been told that the last 3 years all sibilings have been accepted - this clearly makes the whole selection process a bit of a farce. If they are going to accept sibilings automatically why assess other children.
Basically after the assessment there are 3 outcomes - you get a place, you get on the waiting list or you get told that Thomas's is not the right school for your child.
We got a place but didn't take it up and I really regretted putting my 3 year old through it - seriously the have so many exams, interviews, assessments to go through in their life and starting at 3 just feels SO wrong!!!
Last poing - when you are forking out over £4,000 surely they should be able to teach all types of children ?
Having said that - great school and great facilities and I have lots of happy friends there.