I do this. It works, but there are some shortcomings.

You have to install the Spotify App on your PC. Adding new songs has been incredibly flaky for me (not recognizing that I've put new songs in the folder, Windows 10). The artists and albums don't show up in the "Artists" or "Albums" part of my Spotify library.

For now. Spotify has removed so many useful features from their desktop app over the years, I won't be surprised in the slightest when they remove the ability to listen to local files.

As a long time user on desktop, I haven't really seen much change or loss of features. The mobile app, on the other hand, tends to be flavor of the month. No longer defaulting to my library and instead the discovery home is probably the biggest annoyance

I'm surprised to see this. The integration of your personal library was totally ruined (for me) several years ago when they removed the ability to "unlink" songs. When you add your own files to Spotify, the files are scanned and sometimes matched against files in Spotify's own database. The trouble is that Spotify is pretty bad at identifying files correctly and will misidentify them and "link" them to a track on the service. Once this happens, you have no reasonable recourse.

To be fair, this is a pretty niche problem - tends to only be a problem when dealing with live music. So for example, if I have a concert recording, and make a playlist of those files for use in Spotify, Spotify will almost certainly replace at least one of those files with an album cut from the service. Now I'm left with a playlist of a live show that has had several tracks replaced with the _wrong_ version by Spotify.

The thing that initially drew me to Spotify was the fact that it made it pretty easy to integrate the music offered by the service with my personal library. I've always been disappointed that this feature was broken and left broken because the vast majority of people don't care about managing their own music libraries.

I've been a long time user on desktop as well. I was a fairly heavy user of Spotify plugins (and maybe "apps", but I didn't understand the difference). Plugins, of course, do not exist anymore. Looks like maybe about four years ago: https://community.spotify.com/t5/Desktop-Windows/No-Apps-plu...

I'm a little salty about them closing off third-party development, but it's cool, they can do what they want with their stuff. I saw the writing on the wall a while ago when they were pushing Facebook integration like crazy -- as they become more and more anti-consumer, I'll simply avail myself of their services less and less until they offer me no value.