probably bad experience mate.
the disadvantage of the "proper teacher":
1. a lack of knowledge in terms of trend.
2. does not know exam preparation strategies nearly as well. (how long has it been since they did an exam?)
3. lack of current resources.
4. overall may be tempted to settle in with the familiar, i.e. 70s or 80s maths teaching styles.
on the other hand, the advantages of someone like me:
1. knowledge of how much preparation is actually needed to get full marks.
2. understanding of the trends of maths in NSW.
3. recent exam experience, and knowledge of how to prepare.
4. much more resources (up to date, and in terms of volume, since we all collect trial papers much more rigorously.)
5. lower costs.
the only drawback, of course, is that we may not be able to communicate to you the things in our head all that well (depending on disparities between our levels). however, having said that, i am having many students, and it is not difficult to pick up the skills required to communicate perceptively.