We're looking at two different scales, RAAus and commercial flying. However who in the world jumped into the left hand seat of a jet without learning to fly?
RAAus can teach you how to fly a 3 axis aeroplane, which is what a jet is believe it or not. So lets not all forget where we came from.
RAAus is a cheaper option for ab-initio training as many don't have the thousands of dollars to start in GA from scratch. These aircraft are no different to fly to your basic C150, C172 or Piper Cherokee. So why learn the basics carting two extra seats around and paying for them.
For those who want to move onto CPL there is a need to move on to larger aircraft, a la my first statement. The regs (Another hotly contended issue) state how many hours count from RAAus to PPL, CPL and ATPL.
To land your first job you will need to have flown something bigger and faster than a Jabiru hence the reason for the regs containing a requirement to complete your CPL in a contstand speed, >120 knot aircraft.
In short RAAus aircraft have their place in the career of a pilot should they choose to begin there. They provide a fantastic starting point to a successful pilot. From RAAus onto high performance singles, twins, turbines and then jets dependant on personal desires. (Then back to RAAus to fly for fun!)