My conclusion is that able kids are rarely challenged in Primary school, they can just coast along. This is the age that will most shape a child's education - There is little wrong getting a tutor to challenge some more, even if they are not taking the 11+
I totally agree with you. However, the challenge that provided quality homework can also be set by the parent not just the tutor. I suppose the child cannot have a tutor for life.
For example, if my son was taught about the area of a shape at school. When he understood the basic, I would set some higher level questions which he had to think harder to find the answers.
There are a lot of resources out there to challenge the more able. As you said, an able child can finish the school Maths homework within 15 mins. Then, more advanced level work can be provided for further challenge.
Of course, I must admit that it also means the parent will need extra time to search the right materials for that challenge.