It depends. There are bands that have good singers in all aspects. Take a look:
Stone Sour - Corey Taylor's voice is great, BUT... his voice was much more melodic and light, more suited to rock than anything in 1994 with Stone Sour, before they got as big as they are now, Slipknot sorta distorted his voice after years of screaming, studio it doesnt sound much, but its much more raw live. Anyway, If you ever download stone sour - tar poo or stone sour - i can't believe. You will see what I mean, I'll probably put them on my site, since they are only demos sooner or later anyway. The music even sonds differemt its not simple backgrounds, drumbeats and a riff thrown in here like you've probably heard from Get Inside, Inhale and other Stone sour songs. Great riffs, sometimes slow music, some times pretty fast, catchy aswell, great vocals.. blah blah... anyway, if it was down to vocals they would have been signed a long time ago but they wherent.
disturbed started off with a growlish vocie aswell, Linkin Park always had melody and rap toghether, Mudvayne started off with growls and then into singing, slipknot started off with growls and screams, then they got into melodic chorus's. This doesnt make numetal I dont think, I think what makes numetal is well... the commercialisation of it.
If you where to put a band like Arch enemy or old In Flames on the mainstream and charts like numetal bands, they would be just as popular.