it's not an excuse; as with xenophobia and racism, these are phenomena which have to be understood and not dismissed. You cannot solve racism by just stating "races don't exist, here's the DNA evidence etc etc" perhaps implying that you're just an ignorant redneck if you don't understand science. Even if said science is solid, some people will not be moved by it because their bias doesn't stem from a misunderstanding of science (or math) but from somewhere deeper, and you don't have to excuse it in order to understanding and use that understanding to counter it.