The reason for many popups is for multi taskers, the ability to copy and paste links into a playlist builder that is its own windows without interrupting the flow of playing songs currently was the main reason for this. Also the same thing for importing, separating sub features from the main application as utility widgets.
As for the columns here is the reasoning
>Date added Playlist subscriptions are coming, so knowing when a song was added to a playlist can help you know how current that version might be; bands or artist can re-release mastered versions of older songs. So its important to know when the playlist was last changed for that song.
>length
You can sort it from shortest to highest, allowing it to play in that order
>user
Soon you'll be able to login on your own account or subscribe from others, people will be able to fork other peoples playlist to edit them, its mainly just for credit so you know who put effort into building a nice playlist.
I do agree for searching though, I will refine it to be on main display not as a popup and replace the main table where loaded playlist are.
As for suggestions, when out of focus the app will display a popup showing you whats next, should I add this feature to that popup?