Hi Ray. There are plenty of reasons why a songwriter needs to learn to perform. Most have been highlighted in this thread. I would say that any songwriter who cannot or will not play his song is at a distinct disadvantage to someone who can. It is also much cheaper to play and sing on your own demo rather than hire someone else to do it. In fact I can see of no reason why a songwriter would not want to perform other than through inability to play or an embarassment about the song's quality. If you can learn to play an instrument and compose songs then you should be able to learn to perform those songs. Not all great artists relied on songwriters many wrote all their own material including most rock bands and folk singers.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055