The point of having an AO rating is so that we don't get ridiculous cases like Fallout 3 being initially banned in Australia because of the use of syringes and pills (the gore was ok, however!). The drug references were deemed too strong for a 15+ rating. With an 18+ rating, however, it's likely that Fallout 3 would have been accepted for release without needing any adjustments to its content.
What's more is that they cited "realistic use of drugs via syringes, inhalers, and pills" or something to that effect in their statement when the game was initially refused classification, which goes to show just how much they actually researched the games they ban. Now, I've never done drugs, but I'm pretty sure it doesn't involve booting up a wrist-mounted computer, scrolling to an icon that says "Med-X" and clicking on it. As far as I remember, neither New Vegas nor FO3 actually show drugs being used. Just an inventory icon, a sound effect and stat boosts/penalties.
Which leads me to another point, did GTA4 even have the drug scenes edited out over there? I remember off of the top of my head at least three cutscenes that showed characters doing various drugs, not counting Little Jacob's countless references to pot. As far as I remember, it was only edited for blood.
That whole system is flawed, but thankfully it looks like they're finally starting to wise up and fix it.