Honestly, unless you're listening to jazz, most people go out to see a band and hear the vocals. You lose an audience, for the most part, with poor vocals or no vocals. Vocals bring it all together. You can have fantastic musicians, but with crappy singing a lot of people won't think the band is that good. And to a lot of people, the singer IS THE BAND. For those of us who play instruments, and play them well, we don't like to face that fact. But IME, that's the truth.
I do some singing, mostly backup, but I sing a couple songs. I'm not that good. Actually, I'm terrible. I'd be more than happy to hand that off to someone else and concentrate on my bass playing.
With that being said, I would have absolutely NO PROBLEM with a singer who doesn't play an instrument. They are bringing an instrument.....one that I can't play.....which is their voice. If they are hardworking and willing to be part of the band and contribute.....I say go for it. Your band only gets better with strong vocals......and if you have an opportunity to perform better with a dedicated vocalist....seems like a good idea to me. I wish we had a vocalist in our band.....'cause while we are all good with our instruments.....our singing sucks and we would be a lot better with good vocals......