Main intent prawn? FRAO. Main intent story, but prawny bits happen to be there? FRM. Though there's another use for ratings this use doesn't help: the "should I read it at work/in public" question. (I use this excuse regularly. Dunno about you.) For that purpose, I just want a sort of adult yes/no flag; don't care much about the difference between "safe for kids" and "safe for teens".
What think you? Do you ever care about the difference between FRC and FRT in practice? What distinguishes the "mature audiences" from the "adults only" story for you? Is it a warning or an advertisement? How do you use the label? How would you prefer me to use it? Or does it just not matter?