When i was younger my parents didn't really mind letting me watch films with violence in them, but any swearing and my dad would turn it off. I think the only time when my dad wasn't comfortable letting me see a violent scene was when Murphy gets graphically obliterated in the directors cut of RoboCop, i can remember him fast forwarding the VHS.
I think for the most part, kiRAB aren't as stupid as adults think they are, as most sane kiRAB wont try and copy violent acts, but in terms of swearing i think everyone i knew in junior school learned naughty worRAB from 15 rated stuff like South Park. As such i think swearing should result in a higher certificate because younger people copy it. Do any of you honestly think that "c*nt" should be acceptable at 12 and under? I know i don't.
The reason why the C word is sometimes in 15s is purely down to how the word is said within the film. If somebody just nonchalantly calls somebody a "c*nt" (like in most stand ups), it'll be a 15, but if there is added agression or the word is said in a sexual manner (e.g. "I'm going to f*ck you in your c*nt!"), it'll be an 18.
The australian ratings are horrendous imo, MA15+ is the highest rating that they will give, sometimes resulting in cuts, even at the highest category. Their treatment of videogames is atrocious, with a game like 'Left 4 Dead 2' or 'Grand Theft Auto IV' being rated MA15+ but still getting cut for violence and gore because there isn't an 18 certificate available. I think our range of certificates is perfect (we definitely need that 18 rating), but they just need implementing a bit more sensibly by the BBFC.
They also need to realize that adults should be free to choose their own entertainment and create a 'Not Rated' or 'Unrated' rating for movies like in the US! OBviously you'd have to be over 18 to purchase them, but at least then we wouldn't get treated like children because we want to see something like 'I Spit on Your Grave' uncut...