Guys,
Are you sure it's an inner engine that is being shutdown? Standard procedure is to shutdown the outer left engine because it's the one without a generator. You can even shutdown both outer engines if you're keen.
As for safety, the requirement is that in the event of a failure you have to be able to restart all your (operable) engines by a given altitude AGL (3000 IIRC). There is therefore a minimum altitude AGL for 3-engine ops and a higher minimum for 2-engine ops.
This is not some dodgy thing invented by gung-ho RAAFies. It's a legit procedure in the aircraft manual.