I hope they temporarily disabled the feature until they sort it out how to make it work with iTunes Match, without having millions of duplicate song versions, if my above assumptions were right.
These two clues give me a little spark of hope:
- You can still add secondary artwork through the "get info" pane. If they really wanted to get rid of the feature, why didn't they make it entirely impossible to add more than one picture?
- Don't know if you noticed: The moment you add secondary pictures to an album, and press ok, while your mac is updating the files, for a few seconds you can see the arrows appearing briefly. When it's finished they're gone again. So the code for the arrows was probably not deleted but temporarily disabled somehow.
On the other hand, it could be that they don't want to create too much fuss about all this, and remove things step by step... To see how many people would miss it really, how many complaints they receive. Or they were simply too lazy too delete all the code for multiple artwork thoroughly.
Check this link:
https://discussions.apple.com/thread/3529592?start=0&tstart=0
In this similar thread Dave Spencer claims he has spoken an Apple Senior Tech Adviser about it, who confirmed the feature was removed. No more info on the question "why" though.
The worst thing is: I'm not even interested in the whole Match thing. I just want the best music organizing software available. iTunes was pretty good so far. Why taking a huge step backwards by not supporting multiple cover art. It was a basic feature!
Bottomline: please people, complain about it at the feedback page:
http://www.apple.com/feedback/itunesapp.html
and/or use Bug Reporter. Let Apple know we don't agree with dropping this feature!