bias is obviously good for use on curves, but there is another advantage of using it for even straight edge...it lasts longer. Remember that "rips" from simple wear and tear normally occur along the thread lines - either on grain or cross grain = you don't get rips on the bias. When you fold a straight grain binding over the edge of a quilt, it leaves one or two threads running right on the edge of the binding the whole way around...and if and when those threads get worn, a rip starts and continues along the edge of the quilt. With a bias binding, there is no one thread that runs along the edge...all threads cross it at an angle. If one of those threads gets worn, that one thread may break from the wear, but you don't get rips/slits appearing because adjacent bias threads hold the fabric together.