I have had two dc take the 11+ in the last 10 yrs, and another is about to.
Like me, most parents, with dc in the state sector, who use tutors do not think of it as 'buying an advantage'. (Private education, with 11+ practice galore, is more of a contender in that respect). More often than not, they are simply trying to make sure that the gaps left by the state primaries are plugged. Buying a way out of enforced disadvantage, maybe.
In my experience, curriculum-based exams assume familiarity with the entire KS2 syllabus by the end of year 5 - not a hope in hell at my dc's school!
The 11+ takes several different forms, and you could argue that a straight VR/NVR exam is the fairest test of raw I.Q. and, as such, requires less 'tutoring' whether at home or elsewhere. However, I know for a fact that in my area, where there are three papers - VR, English and Maths, and a primary school which ignores the selective system in which it is operating, tutoring of some sort is absolutely necessary.
Yes, I could do practice papers at home, if I had studious, compliant children, who were eager to do extra-curricular work for the love of it
and if I knew how to do all of the maths myself. But guess what?.....I don't!
I do think the 11+ is seriously flawed, in particular the curriculum-based exams, which tend to preclude vast numbers of the very children the Grammars were originally designed for, from accessing them. I also think that the 'opt-in' nature of the exam in many (if not all) areas acts as another barrier.
I don't know what the answer is, other than to revert to the original 11+ format with Primary schools, in areas retaining selection, themselves administering an I.Q test to all children for which minimal preparation is necessary.