The US system is universally loathed by film-makers. There's allegations that the MPAA favours big film-makers over independents, that the MPAA is homophobic, sexist and more welcoming to violence than swearing. The NC-17 spells commercial death, but the 18 doesn't in the UK. There's no accountability in the US, they don't even publish guidelines as to what is acceptable at each certificate (hence the allegations of sexism and homophobia).
The further away for their system we stay the better.
I would however suggest the introduction of a 13 certificate for films that are too strong for a 12 (like the Hunger Games, Chronicle, The Dark Knight), but aren't really 15 rated films...