Well, since computers and the internet are everywhere now, it is nice to have a "human touch" to diagrams, so the hand-drawn look is becoming attractive in a world that is too perfect.
But the designer of the wireframe shapes that Geert posted said that the hand-drawn look serves another purpose: it lets viewers of the document know that this is a design, a rough-draft, a prototype. It doesn't exist yet, and still requires discussion and feedback before it is final.
I thought that was interesting, well said, and an important consideration!