I suppose, if a child is really failing in a subject, tuition could help to salvage the child's performance in crucial examinations. Beyond this fire-fighting assistance, is there really beneficial for a child to have extensive tuition even for subjects he can manage on his own?
Since time and resources are so precious, there are inevitable trade-offs for the benefits that tuition may offer. More tuition means less time for family bonding, for exercise, for pleasure reading, and for interacting with friends and growing in socio-emotional maturity. One may gain a little in academic performance but it seems to offer little else in the way of holistic personal development. If anything, it encourages a "crutch mentality" and excessive reliance on others' help. A child may have gained higher marks but has he really become better at learning?
I am pretty sure that entrepreneurs, pioneers in all fields, didn't get where they are because they had fantastic tuition. Rather, they probably had varied experiences, rich interaction with people, and occasions to put themselves to the tests, and yes, opportunities to fail.
Rather than trying to chase the extra point, the "plus" in the B, it may be better for the child in the long run to have the chance to become a more resourceful and resilient individual by relying on himself to improve his results.