Yeah the people who say that year 11 doesn't really matter are, 'halfway' correct. Most of the things you learn in yr 11 are not tested in year 12. If im not mistaken, none of the english and humanities content goes into year 12. Only a handful of equations and techniques in the sciences are essential for year 12. 30% of your HSC test in maths is prelim work. I didn't do any languages, but Im certain that prelim work is essential for year 12.
I say the above because, from my personal experience, the actual content of the preliminary course doesn't have a massive bearing on your year 12 final result. I did only the bare minimum to pass in year 11 and only began to earnestly learn the content by year 12. I got a result which is moderately good for my standard (atar=95)
What does get carried over are the skills and habits you develop in year 11. You need to learn to listen well, know how to take notes, the proper techniques for answering exam questions, how to highlight and summarise vast passages and develop a routine. You also need to develop your ability to cope with time micromanagement, exam anxiety, distractions and procrastination.
To gain these skills you're going to fail many times and make heaps of mistakes. Thats why you need to try, and you need to care about year 11, because you want to make these mistakes early on when they don't have disastrous consequences, and you want to carry those lessons learned into year 12 from the very beginning of that year.
Oh, and one more thing. If you've set your sights on a particular subject at uni, then you really should study in year 11 for the subjects that are relevant to that degree. Neglecting year 11 content might not hugely affect your HSC marks, but- and i'm rather stating the obvious here- it will affect your knowledge of the subject.