The 11+ just determines which school is most appropriate for the individual needs of your child from a test on one day of their lives.
My advice is beware of forcing your child into a direction that may not be suitable for them. If they had the natural capability they wouldn't need to be coached at all.
I think you make some good points but the fact that it is all dependant on one (or more usually two) days is perhaps part of the problem.
I think that many parents of reasonably capable children are very concerned that one off day (or nervy day) can ruin a bright child's education, particularly if the non-grammar alternative is a particularly poor one.
A little extra coaching may be seen as an insurance policy to ensure that the child has something in hand in case the actual day goes badly.
I think the alternatives dictate how crucial the 11+ is - and some are in a much more fortunate position than others.