I look on tutoring as being like coaching for a child who can run fast , or catch a ball well..just famniliarising them and honing theor technique so that they can acheive their potential
I like the comparison, Yoyo
As for boosting inappropriate children, it is inevitable that some parents will make mistakes, like for any other area of life.
Parents can also find it be difficult to know their child ability because many primary school won't tell more than 'your child is doing well' to anyone who is not struggling!
In addition, there is some research from the NFER that says that grammar schools benefit the most to borderline children. Kids just above the line will benefit, but those just below may struggle. It may be quite difficult for some parents to know what to do, and not all of them have good tutors able to advise wisely.