Actually, I had a look to figure out why MetadataSource is ignoring customization_help() -- it implements it's own config_widget(), config_store() etc., disabling customization_help(). Cool, the config_widget() approach might be just what I need for my plugin.
As for not forcing users to get a key of their own -- there are definitely users who will appreciate that hassle-free setup, and I'm very far from suggesting you take that away from them. For me keys aren't that important (e.g. I signed up with isbndb only to get a key for your isbndb plugin), but other people might feel otherwise.
Still, that shouldn't be an issue -- plugins can be easily swapped. For what it's worth, I think Calibre should always ship with your "easy" plugin enabled by default, and I can just add my plugins on top of that. At some point it might even be possible to push them into the main tree (disabled by default)? I think the real issue ATM would be the quality of my code, not what I'd like the plugin to do...
