I myself have used math to varying degrees during my entire education as a Sudbury student and am currently studying more advanced math for the SATs. If I ever think that my current math abilities are insufficient I will learn more math.
When students are allowed to choose when, how and to what degree they will learn math, they come to view it as a skill which can be improved upon at any point and not something that they are inherently good or bad at. They are also free from many of the negative associations of their traditionally educated peers. Who do you think would be better at continuing to learn math? Someone who is comfortable with math and learned it organically or someone who was forced into learning math and has negative feelings towards the subject?