Here's an example to back up what I'm saying.
Consider the following lyrics from Opeth's "Serenity Painted Death":
Darkness reared its head
Tearing within the reeling haze
Took control, claiming my flesh
Piercing rage, perfect tantrum
Each and every one would die at my hand
Choking in warm ponds of blood
At last, weak and torn, I went down
Drained from strength, flickering breath
Imagine Mikael Akerfedlt using his "nice singing voice" for this part. It would sound fucking ridiculous. The lyrics, the message, the music being played, all DEMAND growled vocals. If he softly crooned lyrics about murdering an entire village while guitars and drums blaze with ferocity behind him, it would sound like a weak parody.