Of course it is a good thing, but they can do it within the same school and then they have the chance to get ahead at any age-their fate hasn't been decided at 11yrs old. Lots of people work out their strengths after the age of 11yrs.
Some DCs are just odd-they don't fit in boxes. My DS was in the lower maths group all through junior school. He was 2nd in the class in a maths exam and the school still wanted him in the lower group-and I agreed. He wasn't good at new concepts because he didn't like making mistakes. On the strength of that he was put into the 3rd maths group at secondary school. He was put up to 2nd after a few months and then we got to the fact that he was top of 2nd group (by quite a long way) but they wanted to keep him there-for the same reason as the junior school. Eventually they were forced to put him into the top group because not only was he top of the second group-he was out performing many of the top group. He got top grades in public exams. He did it when he was ready-he wasn't ready at 11yrs and yet with the 11+ he would either have been stuck with not having anywhere to progress up to or he would have passed and had to cope before he was ready.
A good comprehensive is just like a two tier system, they just happen to be in the same building with the same uniform and they can move up and down as they need or when they are ready.
I know DCs who have been drilled tutored for grammar school and then once there they have had to have remedial maths and english to get them up to standard.
If they need this they shouldn't have been there-they stole the place from someone who wasn't over prepared for the place.
I wouldn't mind if it as a level playing field and they had an test which no one could prepare for-but they can't and pushy parents play the system (and win).
Luckily there are lots and lots of places where there are no grammar schools at all and they will not come back. No one ever says 'I want a return of secondary moderns-I want my DC to go to a school where all the talent has been creamed off'-and yet that is what it means. They mean 'I want the return of a grammar school and I want my DC to have an elite education'.
We all want the best for our DCs. That isn't necessarily an academic education but those with a technical or practical bias want the best too and there is no reason why they can't get it in the same building.
My DS2 isn't academic-didn't go to university-and yet is doing very well indeed, I am very proud of him and I think that ultimately he will do better in life than his brothers, who did/do go. He has plenty of drive and ambition. He also has, as friends ,the more academic type who are at university. His girl friend is at a Russell group university doing a science subject-why should they have been at different schools?