Ok, there's been a lot of discussion already, so I'm just going to post five reasons why you should do the PhB:
1. You will have access to research opportunities normally only afforded to postgraduate students. It's not like you can't do research as an undergraduate generally speaking, but PhB allows you to do it as coursework, and because of your status academics will also be super keen to take you up on your ideas, your prospective projects, etc.
2. You will have amazing flexibility compared to everyone else. To quote the admin people at ANU, the PhB is basically a "do whatever the hell you want" degree - choice, choice, choice is key here.
3. You will meet the cream of the Australian crop. There is a PhB camp in first semester and PhB often has networking/social events - if you are part of the program, you will be constantly engaged with fellow students who have similar intellectual interests and capabilities. This is as useful and as enriching as it sounds.
4. You will have a much more pretigious degree for postgraduate entry than pretty much anything else offered in Australia. PhB guarantees you good grades and a lot of impressive opportunities which look amazing on your resume.
5. You will enjoy it. Much of the problem with the Australian university system is that our universities are crowded, students and lecturers are disengaged, etc. PhB is small, and there is a lot more familiarity between students and academics normally when one is enrolled within the program. This will change your university experience. Trust me.
Seriously, do it. Best choice ever.