Yes, you do need to know how to sing to be in a Metal band. At least if it is a good Metal band, which use both normal and harsh vocals together to make awesome music, I can do harsh vocals really well because my dad said this to me when I was like three years old or something.
"I used to be able to make really cool noises when I was a kid, but I stopped making the noises after awhile and I cant make them anymore, so don't stop making cool noises" not an exact quote.
But the point went across and I can roar and shit like an animal now because I never stopped and it really helps with the harsh vocals as they don't strain my normal voice, but my normal singing voice could use some impovement because good Metal bands incorporate both regular vocals along with harsh vocals.
The song "Happy?" by Mudvayne is an excellent example, along with every Breaking Benjamin song ever practically.