Pete wanted the other members of the band to interpret the songs however they wanted. On the demos, he'd play all the instruments (and vocals) pretty blandly, so the guys would have just the basic framework on which to build.
Pete's lyrics resonated with me when I was a teenager, and now, in my mid-50s, they're just as relevant to me as ever. He perfectly captured the joy/anger/love/disillusionment/hope/fear of life. Quadrophenia is nothing short of brilliant; a complete masterpiece of lyrics, music, and performance.