It makes perfect sense. But if you want all your music in one sidebar-less window, select all the songs in your Music Library, choose File>New Playlist From Selection and double-click the new playlist, then close the original iTunes window.
If I double-click the icon of my Top ~20 playlist, that playlist is opened in a new window. That window doesn't have the library sidebar:
If that wasn't what you were after, then what are you looking for. (The droplet in the top right corner, which removes the Finder sidebar doesn't exist in iTunes, sadly you might say.)
Edit: Too slow with the screen shot... time to go to bed...