It turns the "just shoving windows around and resizing them" from something normal into an act which only a surgeon could feel comfortable with. The mouse is not a scalpel, it's a tool, and if I need to hit a 1px wide border edge (or even worse the corner) in order to resize a window, then it's a terribly bad UI.

I loved SGI IRIX's brutally bold borders, but those of Windows 7 are still just as good.