The downside being that the duplicate detection is nonexistent - I have a playlist I sync to my phone, and often when I'm listening on "I'm Feeling Lucky" mode at my PC a song will come up that I like and I'll add it to the playlist. As you can imagine, this often leads to multiple versions of the same song taking up valuable space. The same goes for thumbs-up and thumbs-down - it won't apply to all versions of a song. Which is fair enough when they're actually different (live, remastered, remixed or whatever) but when we're talking about the European release, NA release and Japan release of the same album it's often literally exactly the same recording.

Trouble is, I can't come up with a "best of both worlds" fix that allows both (perfectly valid) use cases.