[DiapDealer]

setmetadataxml() is for updating the metadata of the existing epub. Since an output plugin can be used to output whatever format a dev desires, there didn't seem to be much sense in providing an epub-only setmetadatxml() method in the output container object. Much like there is no epub-only getmetadataxml() method in the input container object. We couldn't guarantee either would work in all situations.

If one is starting from an epub and ending with an epub, it would make more sense to me to use an Edit Plugin to modify the metadata. If one needs to preserve the original, then Save As and/or Checkpointing would come into play. Using the output container to create a new epub with slightly tweaked metadata seems counter-productive to me.