In order to create a rectangular floor, dance ALL the way into the corners, where you are protected on two sides, and maybe have a little room for a fancier move. If you cut the corner, you have just eliminated a space that would have fit in 2 or 3 more couples, then the next lane knocks of a couple more... multiply times four corners, and you see this has a lot of impact.
(from argentine-tango.com (http://www.argentine-tango.com/dance/index.php?n=Main.TipsForDancing))