In my university PhDs are assessed purely on dissertation, there is no coursework component or oral defence or anything else (this is not unusual for Australia, as I understand).
There are no units for PhD students, but they can get into Masters, Honours or Bachelor level units, assuming they meet the conditions or get exceptions.
However the federal government will pay for up to 4 coursework units to be completed over the first 3 years of the PhD. They will outright pay the course fees for the units -- which is a nice change from undergraduate where the fees go into your HECs debt.
Some advisors/departments strongly advise students to take particular units (though not normally 4 as I understand it). I doubt mine will.
So I guess the purpose of these 4 "free" units is to let shore up any areas of knowledge -- particularly for students embarking on something multidisciplinary.
But what is the advantage in actually enrolling in the units? the alternative to enrolling is to go up to the lecturer and say: "Hi, ... I'm doing a PhD on X, and I know this course covers Y which is kind of relevant to it, do you mind if I sit in on your lectures?"
I suspect more lecturers wouldn't mind at all, so long as you didn't cause trouble -- they are paid the same either way. If you were extra friendly (say you'ld taken some units with them before), you could probably convince them to let you have the digital course notes, and maybe even to mark a assignment so you could see how you are going.
The Advantages I am seeing to not formerly enrolling are:
- No pressure to do well in exams/tests
- Attend only the subset of the unit relevant to your research
The Disadvantages I can see:
- 'Robbing' the university of income (but also not generating any extra expense for it)
- Less access to resources, like online notes
- Not having the unit on your transcript
The Pros seem to outweigh the Cons, but I suspect I am missing something. Perhaps lecturer pay is generally directly proportional to number of students?
I have gone through the (large) university policy base and have found no rule forbidding this, except if there is a shortage of seating.