I think the same point could be made about all sorts of things that are taught in school. As it happens I continued with Maths after grammar school and frequently used things such as quadratic equations which other students may never have used again. There were lots of things I never used again directly as far as I know, such as being able to label parts of the heart, describe the process of photosynthesis, describe the various parts of a castle or analysing a Shakespeare play- but maybe others found it useful and ended up working in a related field or developed a love for the theatre.
At a grammar school you would have covered solving quadratic equations. The majority of students though do not solve quadratics at secondary school as it's not covered in the Foundation Maths GCSE. Many of those taking Higher GCSE Maths will go on to study Maths at A level and will need to be able to solve equations.
I think the point about 'training your mind' is valid. The ability to analyse problems and use a range of strategies to solve them is an important life skill. You probably used these skills throughout your life.