[AlPe]

The plugin developer might want/need to access a couple of basic ebook metadata, like dc:identifier or dc:language.

At the moment, she must call bk.get_opf() and parse the resulting blob.

Although the above requires half a dozen lines of code (if you know lxml and xpath), I am quite sure that this a common situation to the extent that a future version of the Plugin APIs might want to include a call, say get_meta(), to get, say, a Python dict() with the metadata from the current OPF.