Adding a single song to the Library makes a lot of mess, because this song appears simultaneously in three main categories: 'Favorite Songs', 'Albums' and 'Artists'.
But many people listen to different artists in different ways. In some cases we save our favorite songs of a particular artist made during their whole life, in other cases we save particular albums, and sometimes we save to the Library a particular song of a particular artist.
And that's where the issue stands upright.
For example, I like Michael Jackson and saved to the Library all his songs I like. It's convenient to open the "Artists" category, click the Michael's picture and play my personal "The Greatest Hits" of his. But I don't need to see his albums in the "Albums" category, since every album contains only several songs. What should I do with a "two-song album" and why do I need to listen to it apart of the rest of the songs I saved to the Library?
Another example is the soundtrack from the "Assassin's Creed 2" video game. I don't need to see Jesper Kyd in the "Artists" category, since I'm not interested in his music made for other games — all I need is the only one album, that's it.
Third example is the track "Everywhere" by Fleetwood Mac — it's the only song of this band, so I don't need to see them in the "Artists" category and their album "Tango in the Night" in the "Albums" category. I just made a kind of a bookmark for a potential new playlist which would include this track.
As for the "Favorite Songs" category, I don't use it at all. If I want a list of different songs of different artists, — I make a new playlist. But why shoud I listen to the whole Library in one single piece, when there are so different songs saved there?
As a solution of this problem it can be a feature, which would allow to save songs to particular parts of the Library. If you want to save a song for a particluar artist — click the icon "Save to Artists". If you want to save it for a particular album — here's the icon "Save to Albums". If you need a bookmark for a single one track by a particular artist — press the icon "Save to Favorite Songs".
Don't kill the heart icon — just make the Library management a bit more flexible for better user exprience.