if it is the expression of what people genuinely believe and think, and what they sing at the end of a hard day's work for their own relaxation, imho it's folk... if it's none of those things but a tie-in with product and spin-off fashion, with attitudes not genuinely held by the singers but struck to shock or impress the gullible (or to disguise the fact that no-one is prepared to put in the time to learn an instrument or to craft lines which are more than collections of basic rhyme) then imho it's a whole different animal...
this isn't unique to music- most of the arts seem to have this discussion, usually because the louder, more in-your-face artists tend to grab publicity or advertising, hence appear more wealthy than the poet with a cardigan or the draftsman... i'm old enough to remember the performance poet debate of the 60s- few would argue now that much lasted on its own merit- fashion changed and a lot of the poetry became irrelevant. this natural winnowing will happen with rap- it's already happened with most folk and is going on with the newer material. Ralph McTell's Peppers And Tomatos? will be sung long after most of the rap has vanished for two simple reason- it's transferable and it's in the best sense touching.
off soapbox