It's called differentiation: Recognising pupil's individual needs, whenever they are struggling or whenever they need their full potential encouraging. I'm sure no-one would be happy with the idea that everyone should be taught the same...?
I love the assumptions this is all about middle class parents: G&T programmes are based on ability not who your parents are and it's often a way to recognise innate ability and application, allowing pupils from families that have no experience of tertiary education to aspire to and access higher education.
It also has a real impact on pupils individual development. Often talented pupils become bored by classes which they find too easy to access. Bored students become badly behaved students, especially as they see the gaps in the system much more readily! Also, if you're in a school with somewhat of an anti-academic culture, then G&T time can be a bit of a haven for bright, marginalised, studious kids. (Who may also have other social/learning needs)
The idea that intelligence mitigates the need for specific attention is painfully shortsighted.