[CSS Intrinsic & Extrinsic Sizing] sizing of absolutely positioned elements

http://www.w3.org/TR/CSS2/visudet.html#containing-block-details
"If the element has 'position: absolute', the containing block is
established by the nearest ancestor with a 'position' of 'absolute',
'relative' or 'fixed', in the following way:

In the case that the ancestor is an inline element, the containing block is
the bounding box around the padding boxes of the first and the last inline
boxes generated for that element. In CSS 2.1, if the inline element is
split across multiple lines, the containing block is undefined.
Otherwise, the containing block is formed by the padding edge of the
ancestor."

http://dev.w3.org/csswg/css3-sizing/#extrinsic-sizing
"less the box's inline-axis margins, borders, and padding."

Should the extrinsic size of position:absolute elements include the padding
or not? My feeling is that it shouldn't for the purposes of things like
fill-available and auto-sizing perpendicular writing-modes, but I could be
convinced otherwise.

Not that we can change it now, but it's not clear to me why CSS2 chose the
padding box in the first place. What's the use-case this addresses?