I suppose a couple of songs on side two might be filler, but I can never listen to just a single song from Quadrophenia without wanting to hear the whole thing.
Have you ever heard anything from Townshend's Lifehouse Chronicles? The first two discs are fantastic, with songs like Teenage Wasteland (an early incarnation of Baba O'Riley) and Greyhound Girl that have never appeared on a studio album, along with alternate versions of songs from Who's Next and other Who albums that demonstrate how sprawling his Lifehouse concept was.
You may not be into it, but his Gateway remix version of
Who Are You blows me away.
Some of the “reject” songs The Who did are great.
Have you ever heard “Melancholia”? How the heck did that not make it onto an album? Perhaps Pete thought it too dark. But it’s now become one of my favorite Who songs.