Being PC is not bad as long as it doesn't get in the way of the truth. Going to a person and telling them "Hey, you're from a certain group that performs poorly in IQ tests", while may be true, is not constructive in anyway, then why say it?
Telling people to get thin is constructive as they can do something about it, telling someone they're ugly or stupid, for example, isn't constructive so I personally choose to avoid doing so unless I have to.