Think on this for a perspective on the purpose of excellence in music and the relative importance of your own skill: most excellent music schools require composition majors to become a virtuoso on an instrument as a part of their compositional studies. The particular instrument doesn't matter, but without mastery of some instrument (or voice) these schools deem the composer doesn't really understand what's possible from great musicians – and what the potentials are for great music. Think about how the view from the perspective of great skill would affect your own concept of music as a whole.
Here's an awesome modern rock song, Something in the way that you are by The Reign of Kindo. Very smooth and jazz influenced, it reminds me of Todd Rundgren. I love to drift off in the emotions of the chord changes – but my wife thinks it's too long.http://www.youtube.com/watch?v=N6N3D21YFd4
If you're going to write songs, learn to play piano or guitar. Otherwise other people will determine half of how all your songs sound.