I can't really say it's a bad idea with all honesty, as one daughter goes to an excellent grammar school, and the other may have more chance of getting in when the time comes if the school can offer more places.
I don't think grammar schools (or the ones I have experience of) currently help with social mobility with those who are badly off. It does, however, help a lot of academically able children whose parents have a good income but could not afford private school, which is a very wide band of people in grammar school areas, have the same education as those who could afford private school.
Ideally I'd like everyone to go to a state comp and for all schools to provide an excellent level of education- no faith schools, no fee-paying schools, no selective schools (though certainly you'd need streaming within the schools) and a lottery system for places to avoid selection by house price.
But one thing the top grammar schools are doing now is countering the effect that nearly everyone influential in society: in acting, music, television, politics, literature comes from a well-off, privately educated background. It's a massive leg-up for those on middle incomes and that's why parents like them.
If you get rid of grammar schools and not private and faith schools at the same time you would be just perpetuating a system where the elite always come from the same people and the rest of us, well, you might get to a middle management drone job if you are lucky. Don't try and have any creative or really influential career aspirations.
This move is a sticking plaster and a sop to voters in certain areas who might vote conservative, and probably a policy tester for the creation of further grammar schools. But at the same time I can't be totally against it as it directly benefits my children.