The implementations notes list another potential specification[1]. I'm guessing it was avoided because it wouldn't follow the pattern of ending each command with the new coordinates of the pen.
They were definitely considering the rounded corners use case; the details of drawing a rect element with a path[2] make arcs sound much easier to use than they actually are most of the time.
[1]http://www.w3.org/TR/SVG/implnote.html#ArcParameterizationAl...