It's fair to say that looking through history that most well known composers were very good if not absolutely amazing pianists (or harpsichordists and organists before the pianos invention). There are obvious exceptions to this statement like Dvorak and Sibelius who were good violists and violinists , respectively, and I believe Berlioz began on the guitar and flute. However, they all seem to have composed for the piano at some point. Even in other genres like jazz and pop the main writers of standards and big hits appear to be pianists or in cases like Mingus played the instrument as a secondary.
So my questions are, how much of an an aid in composition do you believe it is to be at least proficient on the piano? Are there any great composers who completely shunned the piano? And is their a difference in the musical style of piano playing and non-piano playing composers?