Can I just point out that you are all looking at this from the point of view of students whereas the governments have to consider the wider view of the state/country as a whole?
One concern that NSW does have to deal with is that it has a very low retention rate among its students from Year 10 to Year 12 in comparison with other states and territories (I know the NT is lower but I think that is the only one). As far as the country is concerned that is not good - about 1/3 of NSW students don't continue on past Year 10 and about 10% of the state don't even complete Year 10. There is quite a bit of anecdotal evidence among teachers that it is the SC that causes many of these students to leave school both before doing it and immediately after it.
In addition having tests in Years 3, 6, and 9 would reduce the amount of state wide testing done in NSW where students are currently tests in Years 3, 5, 7, 8, and 10 (from 5 testing years to 3 testing years).
NSW is the only state with an external exam in Year 10 and yet the students in other states, particulary Vic, with the VCE, seem to cope very well with the stress of that exam without the experience of a Year 10 exam.
QLD don't even have an external exam and they have a very high retention rate.
Is there a message for governments there - take some of the pressure away and keep kids at school for longer and off welfare and out of trouble.