Roy of course some are better at certain things than others......some people write great lyrics but are rubbish at writing tunes...and vice versa. Lyrics follow a musical pattern and meter etc. You cannot write to any musical pattern if you do not understand MUSIC.
To say that lyricists do not need any musical training or knowledge...is an insult to all songwriters whether lyricists or musicians.
I also think it is ethically dubious to claim creative and financial rights and rewards for work that is clearly in the main done by others. Sadly this seems to have become accepted practice in the industry as needs must for starving musicians.
The music history books are littered with musicians who were the real force responsible for a hit but only got paid a pittance on a work for hire agreement.

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