As if requirements are set in stone.

The parent comment is a bit exaggerated but it still has a point. One needs to be able to predict things and make sure whatever you do today isn't so rigid that it can't adapt to other things in the future without a major overhaul. Typically this means being cognizant of the assumptions made along the way, and making the minimal set of them. This kind of insight requires experience. People preaching YAGNI just don't seem to have much of it. They just read "YAGNI" in a blog post and then yell it at other people.