It seems this NEA Opera Awards thing has really hit home for Philip Glass. There's been more than one article of him getting choked up at the ceremony. Here's a very interesting quick Q & A that he did with Expressnightout.com.
When some dust has settled at the end of Philip Glass' career, I believe his assertion that he's first and foremost a "theater" composer will be true. I don't discount any of his concert music, everything from the Violin Concerto No.1 to Symphony No.8, but I think it's hard to find a musical accomplishment as deep and moving as Satyagraha in the last 50 years. Consider other magical moments in the catalog including Koyaanisqatsi, and I think in Philip Glass' mind it's all mostly an extension of his theater music, including the first Violin Concerto which is essentially a dramatic work.
There are many reasons for this. I don't think that all of his music is "theater music," but rather his music works best when applied to theatrical puposes. Godfrey Reggio knew that Philip Glass was his man when he simply took some of Glass' music and put it against his film and said "See, it works!."
In fact, I go further and think that Glass' best music is absolute music in the best sense: Glass composes the music – it has its own logic and structure… then it is given to others to make something of it. I mean this as it's an inspiration for directors, writers, performers…everyone. By contrast I think when Glass is given, for example, a comedic movie scene and told to write appropriate and functional underscore, he can do it, but the music suffers from strangulation and limitation and ultimately doesn't work so well.
But here's to staying true to your voice. I'm just happy that, as he says, he was taken in by the opera world. Without the invitations we wouldn't have Satyagraha, Akhnaten, Waiting for the Barbarians, the Cocteau trilogy, Civil Wars, and many other inspired scores.