For myself, the only parts of the metadata I am interested in is covers, book summaries, titles, authors & series/index. I don't have any tags in my library. Given my rather limited reading interests (SF, Fantasy, Biography, Electronics/Computers, Photography & Food), I don't bother with genres either. It's hard to mistake an AMD CPU guide for a southern cooking book.
When I first started using calibre and added my Baen, Gutenberg, etc. ebooks, I downloaded metadata individually. I remember being limited to Google and Amazon (at least going by my memory, those were the two sources) though I'm not sure if they had bulk download capability. Later I expanded to using other sources when calibre got it's plugin interface and metadata pugins plugins started to be created.
I really don't agree with trying to limit people's ability to try bulk metadata downloads. It will be nothing but a headache when people complain about not being able to download metadata for a thousand books at a time. In my testing, most of the sites I tested recently blocked me after downloading metadata for between 4 to 10 books.