saying "races don't exist" because there are ppl who can't be id-d 100%, is like saying "white and black colors don't exist" because there are transitional colors between them on the palette. just because there's a poor sob amstaf and poodle mix puppy somewhere doesn't mean there are no poodles and amstafs. etc etc.
Social is one part of it, sure - just like it is for any ethnicity. That's the cultural aspect of ethnicities. But race is still important. Wouldn't you think it's a bit odd if I went around calling myself an african american when I have absolutely no connection to Africa whatsoever? Eminem is embracing african american culture and mannerisms 100%. But he never goes around calling black people 'niggas' which he obviously could have if he himself was black.