I think someone in the band needs to write them, not necessarily the singer though. Some people have amazing vocal talent, but crappy songwriting skills, so they pair up with a great lyricist who can't sing.
But I think as long as the music sounds good, it's all good. Because honestly - if you heard a song and loved it, if you found out later that someone else had written for the person would you hate it then? That's pretty stupid if you ask me.
However if you heard a song and loved it, then later found out that the person or person in the band wrote it over a tragedy or a loved one or something, well, that just makes it an even better song.
Edit:
"Just" a singer? Jesus, people spend their entire LIVES devoted to training and practicing and perfecting their voice. To call them "just" a singer is like saying, "Oh, Beethoven? Yeah he was just some piano player."
Edit:
"The worse thing (to me) that a musician can do is to rely on a professional song writer. This tells me that they have no creativity for one, that they have no real love of music, and that they are solely in it for the purpose of making money."
Wow. I'm speechless. So it doesn't matter if they spend years creating music for their next album, if they have someone write a song or two for them, they suck huh?
So basically a person MUST be talented with their instrument (which includes vocalists) AND they have to be amazing songwriters, and **** whoever can't be both. Damn, I guess that means some of those idiot-savants aren't talented. Sure they can play beautifully, but since they can't write lyrics they don't deserve our respect, huh?