Sometimes music is deeper than what your shallow little minds can comprehend.
First of all, if a clean guitar can be music, a distorted guitar can be music, a drum, a trombone, the sounds of birds chirping, a piano, the sounds of waves... then why the hell not a heavily distorted voice? Just because it's not the 'normal' doesn't mean it's not music.
And it is in fact extremely difficult to do. I know a guy who practiced all through highschool before he could do it somewhat good.
Thousands have proven that it's possible to show emotion with the 'typical' singing methods..
Now try doing that with growls.. seriously.. record some guitar tracks and try growling some lyrics over it.. record it. Listen and tell me that it sounds as good as Opeth's growl vocals.
I've tried it, and gave up on it within a week. It requires too much devotion to be able to do.
Stop trying to listen for words when your hear growls.. listen to it as another instrument. Just the same way people probably neglected the first distorted guitar you are neglecting distorted vocals.
Now, I will say though, some death metal is complete shit when it comes to vocals. Like they are doing it just to be 'death metal'. That's not music... that's trying to be an image.
But the few bands who do it as another way to express their musical ideas... \m/