I'm a classically trained pianist and teacher who has spent the better part of the past two years delving into the world of blues- particularly the piano styles that came out of New Orleans from people like Professor Longhair, Fats Domino, Allen Toussaint, James Booker, and Dr. John. Let me tell you that I find this music, which appears quite simplistic on the surface, to be completely fascinating once you really get into it. While the basic harmonies in blues are extremely simplistic, the best players substitute, embellish, and voice their chords in the most creative ways- ways which often leave me scratching my head and listening to the same passages over and over asking myself- "How did he do that?!" Rhythmically, the piano boogie and blues from New Orleans is amazingly complex and takes considerable technique and practice to master. African, Creole, Cajun, and Caribbean influences can all be felt.It's true that much "pop" music is rather trite and is not especially effective when performed on the piano. But there are certainly styles of popular music which deserve to be respected as 'legitimate' genres for piano players right up alongside the European masters of the past.
I don't think it's really either/or or even learn this/then this. There's SO much music out there, it really only makes sense to me to play the music that you love. Working on classical music definitely gives you a technical foundation that will help you when playing all types of music. Working on music from a classical musician's perspective can make you a better sight-reader and enable you to become quite proficient at reproducing music composed by someone else. All very worthwhile and beneficial skills.Playing solely from a classical musician's perspective means that you are probably not spending much time creating your own music- either through improvisation or composition. You probably aren't doing things like developing your ear to be able to play what you hear in your head or exploring different accompaniment styles.There's no reason at all why piano study can't involve "classical" and popular styles of music simultaneously. I for one find myself more motivated to practice when my practice sessions are an ever-changing combination of repertoire and skills. Today, I might be trying to memorize that last section of a Bach fugue I've been struggling with. Tomorrow, I might try figuring out some Dr. John solos by ear. The next day, I might try taking a pop tune I've been playing for a while and working it up in a different style. Then, maybe it's on to sight-reading some Chopin.It's all good! Happy practicing!