It's not even about being diplomatic or being less likely to hurt someone's feelings (although you get that for free), it's really just about clarity as to what's being discussed. The most confident, secure person in the world might still get defensive when confronted with "you're wrong", because there's still a whiff of it being about questioning that person's credibility, as opposed to the thing that you think they're wrong about.
That together with the old chestnut "seek first to understand then to be understood" to ensure that you respect and understands their position are real game changers.
Its funny and scary how important these kind of social and interpersonal skills are working in any corporation. Yet often are lacking and leads to a lot of unnecessary friction.
It's something I almost wished was taught in schools, if only so they could recognize the patterns later when they go into working life.
http://www.slate.com/articles/double_x/doublex/2010/03/can_y...
But in all seriousness, projecting issues as character flaws is a really destructive thing to do, not just in programming but in any relationship. It's a really good habit to avoid it whenever possible.
I read somewhere (I think it was Blink) that marriage counselors could tell which marriages would fail by observing those that generalized around their spouses character rather arguing about the issues, ie "You always..."