I have always been very frustrated with the fact that if I want to listen to a song or use it in a playlist (offline) within the Spotify app, I cannot do so with a song that is already on my device.
I use the Spotify app on my iPhone for most of my music listening. However, I have a handful of other apps that allow me to listen to my local music through them (like a game or workout app for example), but all these other apps can only see songs on my iPhone that were synced via iTunes. Say I want to listen to a certain playlist or album during my workout using my workout app, but I also want to listen to those same songs later in the Spotify app (offline, to save on data streaming). This would require me to have TWO copies of each song on my device (Which is rediculous! Who has the extra space for that?), one in the native music player library, and one synced using Spotify.
PLEASE find a way for the Spotify app to recognize the existing songs on a mobile device without having to sync them using desktop Spotify. No one should have to have duplicates of local songs on their device. Compatibility with the local native music on a device would be a godsend!
Something Spotify is missing is some sort of app or a part of spotify itself that allows users to see how similar their friends musical taste is to theirs. An app that could show this data, and also sort that list of friends by how compatible they are with you or other people would be a great innovation to Spotify. As it is now, the only way to gauge this is to actually look at what songs their friends listen to.
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