I know that most musicians who attain AMus are decent, but the grade that someone's accomplished isn't always indicative of their talent. I know some people who passed AMus who play much worse than their counterparts undertaking 8th grade. In saying that, I also know some people who've attained AMus who produce a beautiful sound.
I still think that it's expensive to pursue extra-curricular activities. Take music. Let's say you manage to get a cheap deal and can rent a violin for $50 per term. Let's also say that you have half hour lessons (which, let's face it, aren't enough for most people to accomplish much) per week for 40 weeks of the year at $30 per lesson, which is the standard rate. That works out to $1400 per year without exams or music purchases. On top of that, most students who wanted to attain high level orchestral experience out of school would have to join Melbourne Youth Music, which costs around $600.
I'm not into sport, but I presume that the costs for uniforms, equipment and tours would add up when people are competing at a high level.
I feel truly lucky that I'm able to pursue my interests beyond the classroom, but I know a lot of people who just can't afford it. It'd be unfair if they were disadvantaged further in the selection process for university. Sure, consider the activities that people do. But don't emphasise it to the extent that it discriminates against those who don't have the opportunity.