[css-shapes][css2.1] <basic-shape> vs. <shape>

Why are there two definitions for shapes? The <shape> value defined in CSS
2.1[1] (and used for the clip property) and the <basic-shape> value defined
in the CSS Shapes Module[2].
Furthermore they both contain different syntaxes for describing rectangular
shapes, i.e. rect() and rectangle().
Shouldn't both be merged into one definition? Or the definition in the
Shapes Module extend the one described in CSS2.1?
I noticed there has already been a related discussion about this[3], though
it seems without clear resolution.

Sebastian

[1] http://www.w3.org/TR/CSS2/visufx.html#value-def-shape
[2] http://www.w3.org/TR/2013/WD-css-shapes-1-20130620/#ltbasic-shapegt
[3] http://lists.w3.org/Archives/Public/www-style/2012Sep/0061.html