I might be able to explain how it came to this confusion.
The DC metadata "Title" used to be implemented by the property "title". Most properties are used for configuration - and apparently, someone wanted to have most simple configuration at a central place: the registry.
title might be viewed both as configuration option as well as content object metadata.
I agree however, that both views should give the same result -- and that "title" should not be maintained in the registry.
In theory, the registry is cleaner and more modern than property sheets interface;
In practice, the registry and access to it requires adaptation to get title (either plone_portal_state or registry);
We could make TItle / setTitle() accessor/mutators front for the registry;
Or we could just keep these in sync with event handlers (but at what risk/cost?).
The benefit of moving back to exclusively property sheets does not likely justify amount of effort (grepping my omelette, I get this sense).
Picking one place to store is likely better than syncing two places;
Site interface ought to still appear contentish (CMF Dublin Core accessor/mutator), even if a facade to various places the "truth" is living for specific fields.