Speaking from experience, my Sec 4 classmate had already made up his mind to go to Poly to pursue computing. However, he still joined JC to check out JC life (%$^&*), and quit after 2 weeks of school.
As a current J2 student, there are certain topics that I feel are not going to be important in my life in the future, but as major topics, I've no choice but to study them.
Consider this: You know that a particular topic is not going to be of help in your future, but yet you are forced to study it though you don't like it. As a result, you fail your exams or do not do well in it. However, you excel in other areas you are intersted in. What are you going to do if you do not qualify for the university course of your choice due to poor results?
Personally, I feel that universities should just set a minimum score that students have to achieve to qualify for the U (e.g DDE), and when it comes to choosing courses, applicants are given n entry test to decide whether they qualify for that faculty.