It will make competition for grammar schools more intense than it is today. To the extent that competition is about buying an expensive house in a grammar school district and spending money on tutoring, it becomes more financially challenging for "poor but high potential" kids.
To the extent it's about extra homework and setting an 11-plus bar for quite young children, which I suspect few people consider intrinsically desirable, that bar becomes higher and the homework more intense, and I'm not convinced that's great news either.
Of course it's particularly "unfair" for want of a better word if (like me) you have a 12-year-old who's extremely bright and would have strolled into our nearest grammar school; if you thought (like me) you'd go to the geographically more convenient private school near your office and leave that grammar place for somebody else, and now you've probably missed the boat.
I'm actually off there next week to see if the grammar school has a place at 13+. Unlikely I know. But if they do then I'll accept it and probably I or Mrs Chips will quit our job - we won't need the second income any more and the school run won't allow us both to work as we currently do. That'll be a £40k hit to the public finances on our income taxes and NICs, plus the cost of the grammar school place. Please can one of the pro-VAT people tell me how that's making the country better for anyone?