If it's A-level further maths, take it from me as a researcher in mathematics that what you're teaching isn't maths, it's how to turn the handle on a bunch of procedures to get an answer. Mathematics is creative, involves thinking about problems in a new way and understanding them on a fundamental level. None of this happens before undergraduate, and precious little even then. Certainly nothing in an A-level - you learn algebraic manipulation, calculus, imaginary number etc, all nice concepts, but rather akin to learning a wide vocabulary; it's a lot of complex words that you know how to use, but what you're doing isn't literature.
This is going to be true of maths and science right up until you are doing it 'properly'. I do explain this to all my students. But they have to start someone and they need the basic skills to get there. You cannot have just the creative and innovative and thinking skills. You need the basic tools as well. Both need to be taught, and realistically speaking it is a lot easier to 'teach' the procedures, basic skills and knowledge at an earlier age than it is to churn out ready made right thinking mathematicans at a young age. It is not a perfect system but having been involved at most levels (teaching young children, secondary school teacher, undergraduate teaching, done my PhD. etc) I can see the problems that everyone has at each stage. We are all going to complain that those sent to us do not have the right skills (I spend enough time moaning that my Year 7s and Year 12s are not coming up prepared) but in reality what would you change about that?
My selling point on Further Maths is just that. It gives you a taster, a little insight at some of the basic skills you will pick up and run with at a higher level. It is not the pinnacle of mathematics and nor should it be. That is what university is for! The next level of education. The original post here is talking about UK Secondary Education, not preparing that tiny handful of students that are motivated and intelligent enough to be able to do things that their peers will take a few years more to grasp.
Doing mathematics at a suitable level, let us say postgrad, will of course give you a skewed view. I have taught last year some Further Maths students, one of which went off to Cambridge. However, at the same age, in the same year group, the same cohort, we were teaching students GCSE Maths for the third time, and some of those still fail to gain any grade at all. Any given year group in secondary school has a massive range of ability, that is quite apart from the quality of teaching, social skills, home support or any other factors. This is what the curriculum has to deal with and it is all too easy for those not doing it to say that everyone should come out in tip top condition ready to roll for that subject for the rest of their life. This is before you consider they have some 9 other subjects or so in which we could discuss the same thing.