The reason there are two camps is because lyric writers generally have no idea as to the amount of work that goes into writing music and recording a song in comparison to writing just lyrics.....if they did their attitudes might change. It is no secret that lyrics can be penned in a matter of minutes.....many hit songs have been written on a bus.......big difference in writing music.
Sometimes a musicator has to almost rewrite the lyrics to get it to meter. Polly made a great point about arrangement and performance......gone are the days when a guitar/voc sold the song........people want full blown productions......and that is what sells the song.....not words and simple melody.......Try to imagine some of the recent chart hits performed as a simple guitar/voc.....it just would not happen.....folk would laugh at the crap words and melody.
I note that lyricists defend an equal split despite knowing that musicians work is harder and more time consuming.....to claim that it takes as much time and knowledge to write lyrics is nonesense. Musicians who contribute a MAJOR part of the song have no entitlement sucks BIG TIME.
I remember hearing about a guitarist who did session work for hire...he was asked to provide an OFF THE CUFF lick during recording a film theme tune.......It is probably the most famous guitar lick ever The James Bond Theme. The guitarist go no extra and no creditation whatsoever......HMMMMMM.
I hope that if any lyricist ever makes it with a song they would give credit and a percentage of income to the REAL guys behind the work.

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