Funny thing is I just had this long conversation with a few guys my age (mid 40's) and a bunch of kids at the School Of Rock (15-17) . We touched on this exact subject and we came to some conclusions, but not an answer. It seems to be partially an age thing and how the youth feel about thier current state of affairs in this world. Me and the rest of the guys grew up on singers like Freddie Mercury, Robert Plant, Ozzy Osbourne, Jim Morrison, Rob Halford, Bruce Dickinson, Ronnie James Dio, Peter Gabriel, John Wetton, Greg Lake, Ian Gillan, Ray Gillen, Glenn Hughes....... These guys were all mostly influenced by a few main genres of music that came before them such as Blues, Opera, Jazz and Rock. Songs that had lyrics were called "Love Songs, "Aria's", "Torch Songs", "Scat". So most of us grew up having our emotions touched by lyrics that were conveyed by these influences. The reason we had this conversation is because we have about 40 kids right now in the school and there's only 3 kids that can actually carry a tune when singing classic rock tunes. There were more kids who came into the school and said "I'm a singer", but when asked to sing Baba O Reily, could not stay in tune, did not know how to change the key could not fluctuate their voice to accomodiate the changes in the melody that was being played, just had no understanding of what techniques are to be used to achieve the desired effect of singing a song as simple as Baba O Reily, but the kids that were singers in their own bands all come from these bands that just scream!
Just reiterating what has already been posted, hostility, agression, brutality, evil, harsh, rage, fear, pissed off.... are some of the words that have been used to discribe why growling vocals are accepted, because they convey these emotions. I think there are many people who feel this way which is why this is such a acceptable vocal "technique".
When it comes to growling, I just don't find any kind of influence. I totally understand that these type of vocals do in fact convey a emotion and may touch people who listen to this music in a very deep and emotional way, but I'm not there. It has nothing to do with how "deep" of an emotion state I need to be in. Beleive me I can get pretty dark & terrible as I have been over the past couple of days, with my 93 yr old grandmother who does not know who I am any more, who cannot walk, cannot speak english any more only Polish and of that most is just gibberish, has osteoperosis so bad that she can break a bone just by moving her, and 2 days ago she fell out of her wheelchair broke her femur in her right leg the ulna & tibia of her left leg a bone in her foot has a baseball sized lump on her forehead. I looked at her 2 days ago and said why?? Then the bombings in the UK, I was so angry, sad, pissed off, but I regress.
The importance of vocals to me, is that I need to understand what is being said in order for the lyrics to efffect me. Boy do I sound like our parents!! That's what my mother in-law says about the music I listen to, she would say "Now Nat King Cole can sing, how do you understand what these people are screaming about"

Freddie, Daniel Gildenlow, Bobby Kimball, Peter Gabriel all have an individual ability to convey different emotions from sorrow, sadness, anger, rage to joy and blissfulness by changing their techiques. I just don't find growling to have any techique except to growl or scream. Iis there such a technique as a soft scream. Now you may think that I don't listen to this music so how can I pass judgement. These are some of the CD's I have in my collection.
Into Eternity - Dead Or Dreaming
Killswitch Engage -Alive or Just Breathing
Meshuggah - Contradictions Collapse & None
Sevendust - Animosity
Thyfing - Urkraft
Soilwork - Natural Born Chaos, Figure Number Five
Opetn - Deliverance, Blackwater Park, Damnation
Lamb Of God - Ashes of The Wake
Hey look at that, you could write a song on the titles of these cd alone!
Dead or Dreaming,
Alive or Just Breathing,
it's just Natural Born Chaos
Filled with Contradictions, Collapse & None
but I regress again!
I just don't find this type of vocal "technique" to move me, but there are some that I do like. The new Lamb Of God is kicking my ass. I don't know what it is, but I can understand what Randy Blythe is saying, Now You've Got Something to Die For! just floors me! In the end I respect that this is a current style and "technique", but one form of art that I don't find appealing. Most of the music that has Growling/Scream vocals just doesn't touch me in a emotional way like Halford singing Screaming For Vengence or Last Rose of Summer.