Setting bug importance to 'Medium' (compliance to the SVG specification is a major goal on the roadmap): even though Inkscape itself does not create such masks (it only uses the top-most (or bottom-most) object of a selection as mask), it should properly render externally created valid SVG files with masks like in the reported case(s).
Also reported and discussed in bug #901198.
Setting bug importance to 'Medium' (compliance to the SVG specification is a major goal on the roadmap): even though Inkscape itself does not create such masks (it only uses the top-most (or bottom-most) object of a selection as mask), it should properly render externally created valid SVG files with masks like in the reported case(s).