Mags, you're a trained musician whose instrument happens to be their voice. :-) Unlike playing an instrument, anyone can sing without any barrier to entry.
The blessing and curse of singing is that the quality of voice is preset at birth, like any physical ability. From that baseline, some significant improvement is possible through good technique, which does really make a difference. But "someone with a nice voice" will inevitably outperform someone who hasn't in the quality stakes, no matter how hard the person without a nice voice tries.
I speak as someone who, through three years of formal lessons and four years of practise with a band, has just about got his voice to the level of "not unpleasant". The problem isn't holding pitch or range, it's simply whether your voice has a nice tone or not. If you were born with a voice that hasn't got that tone, the fact that you have two-and-a-half octaves of range, ears like a hawk and musical theory spouting from every orifice is immaterial.
So if you want a good singer for a band, find someone with a nice voice and coach them on the musicianship side. You can't get there by taking a musician and coaching them on the voice side.