If its been your dream to sing, then would you really be happy otherwise? Now, if you downright suck, practice on your own time to better your voice. But, there have been plenty of mediocre singers who made a large impact and are some of the most beloved bands.
So thats my take, is to consider your happiness. If your friends abandon you for this, fine. Nothing wrong with two bands surfacing from this situation, right? After all, you shouldn't disappoint the kid in you.
BUT if you really dont give a hoot about singing, then yeah let the sucker sing. I'm all about backing my brother since his voice blows others out of the water, and I want in on that fame. After we get big then I'll consider fronting my own band. So you could look at it that way too.
As for lyrics, I would not feel comfortable with someone singing my lyrics. However, my brother writes exceptionally well on his own, so it worked out for me. I cant imagine dealing with a singer who has nothing to contribute lyrically. So I cant give you advice on the lyrics dilemma, but I hope my above insight helps you some!