I'm glad someone asked that question, because I've wondered that for years. When I first made the decision to go head-on with songwriting, I picked up a few books to try and familiarize myself with the processes of the business. One of the books was by Jason Blume, and I recall a section about his early days in the writing biz...he didn't play an instrument, but he still sang his melodies into a tape player, even though he says he was far from a singer.

I see something like that being, at a minimum, a requirement for a songwriter. You don't have to sing well...you don't have to play well...but I think if you don't at least attempt to put the words to music...even if only in your head...you'll never really get a feel for how / if they flow.

Just my two cents.