It seems strange to me that one can't paste but can cut/delete from locked layers: I'd have thought that one is usually more concerned about preventing destructive changes than additive changes.
I would word this differently. What is made difficult is _accessing_ the locked layer. Both selecting and adding to are ways of accessing it, and they are now both equally disabled. Deleting objects from a layer, however, is a next step which only becomes possible after you access the layer. So, if you do find a way to access a locked layer (e.g. via XML editor), changing it in any way (deleting or adding) is equally easy. I don't see anything illogical here.
As a developer, I understand that things are this way on the principle of "allowing changes to objects once one has gone to the trouble of selecting an insensitive object", as I myself wrote (albeit in a different context, e.g. on the understanding that insensitive meant merely not canvas-selectable rather than locked). What do people think about this now that we're interpreting insensitive as meaning "lock" ?
I think "lock" is a good term here. A locked layer is like locked house; you can't normally enter it and move things around. However, the inhabitants of the house are not dead or petrified, they are just locked away from you :)