What about bulk additions? The existing edit metadata tools only let you replace files in one item at a time. Matches might not be perfect, but in the cases where there's one and only one exact match to a title (or title/author combination) we can be pretty certain which item to replace/add to.
I imported hundreds of ebook files (all legally acquired!) from Baen before I realized that the format I'd chosen didn't have a table of contents. Many of these now have series information now entered into the metadata.
On one hand, serves me right for not testing more carefully, and frankly I'll have to do most of the work a second time just by *downloading* all the files all over again (which I'd have to even if I'd saved files instead of deleting them after seeing that they opened within calibre -- the .lit files don't have a TOC, or at least not one that imported successfully, where the .prc/.mobi files do.)
On the other hand, I'd rather not start reimporting them from scratch if I don't have to, and even a thoughtful and experienced user might decide to reacquire multiple ebooks in a better source format if they became available at a later date. (Possibly, in the future, because a better source format starts to exist!)
On the third tentacle, checking out the source myself has to be a good thing, even if my experience with python was in much smaller projects.