I'd have thought that the main purpose of sodipodi:insensitive (at least as applied to ordinary objects other than layers) is to allow selecting things under the insensitive objects rather than to prevent them from being modified.
I'm not very familiar with using layers in drawings, though I'd have thought that the same would apply to layers, to allow editing lower layers.
Once one has selected an insensitive object somehow (Select All, Find, etc.), I'd guess that one would want to edit it (though I'm not an artist, so ask others). A separate read-only attribute might in some circumstances be useful, though I'd suggest that it not be in 0.40, and I suggest that insensitive is more useful than read-only.
The naming and icons for insensitive is another issue (see "Lock vs Sensitive/Insensitive" thread).
pjrm.