I've been having little ongoing bits of discussion about this lately, so lemme see if I can sum up.
It's not about being PC. PC is a term, uttered with a sneer, by shitheads who are peeved that they can't go around being offensive without consequences.
It's about not being one of those shitheads to begin with. You may not know it, you may not get called on it, you may get an uncomfortable laugh or a stunned nod from someone who just made the connection in their head that you are not a good guy because you use language that shows some little hateful side of you.
And I get that removing those old, relied on phrases and terms is a hassle. I get that the extra seconds it takes (in the beginning) to find a non-shitty way to say what you're trying to say makes you feel a little stupid. Hell, about every 6 months I do another "I need a new insult" post just for that reason. But I do it because I don't ever want someone around me to think I'm not someone they can trust simply because I'm too lazy to use my words carefully.
I never want to wound someone with my words on accident (though I am more than happy to do it on purpose). I never want to accidentally insult a whole group of people when what I am really trying to do is convey the awfulness of one person. I never want someone to trust me less because I can't be bothered to use words that don't hurt them.
Not using oppressive language is something I try to do because I don't want to be an asshole, an accidental asshole. That's all there is to it.