What rule says that songs have to be cowritten? A great song is a great song regardless of how many write it. I am sure that any producers or artists are only interested in the song fitting their needs and not how many writers were involved.
Why is it always assumed that all roads lead to Nashville? There are other genres and other places that produce huge hits. I am sure the rules are pretty much the same regardless of where a song is pitched. The crucial thing are to produce a quality song that is suitable for the target market, know who to pitch it to and produce a demo that sells the song and you. If people put out second rate songs with third rate demos then that is to their disadvantage and my advantage if I am competing against them.
Ray might be interested to know that the big boys can do pretty much what they want.....They make the rules and all the decisions.....songwriters just have to comply....or suffer the consequences. In order to play by the rules you have to know them first.

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