There is a spectrum known as emotion, in it, there's a diverse range; ranging from happiness, sadness, desperation, anger, and many others if you want to get into the technicalities of their definitions.
I started with Metallica about six years ago. I remember I didn't like his vocals (Ride the Lightning) because he was sorta screaming and I thought that was stupid. But I liked the music, especially the solo's, and second came the riffage (which is probably my favorite part of Metallica now). So with enough patience listening to the music, I grew acustomed to the musical style, and started singing along.
A few years later, when my musical diversity increased, and I listened to more thrash and misc. metal, I found In Flames. I was absolutely appauled by his vocals, I really couldn't grasp death metal vocals; but the music was good enough for me to keep listening to. I ended up getting every album, and by the end of it, once again I found myself singing (or screaming) along.
By the time I found Opeth, I was ready for it, and at one point it all clicked. I think it was when I read the lyrics to Still Life. If you follow it closely, clean vocals represent a sense of calm or hope, as opposed to growling vocals representing anger, desperatism, and other darker themes. And that's when it clicked, growling is not an immature form of some punk-wannabe assholes trying to be different in the name of idiot-teenage rebellion; it is as valid a form of art as Beethoven's 5th symphony. I mean, looking back in history, Beethoven's 5th is off the fucking wall; if you listen to the entire first movement (approx 15 min), you'll find it's one of the most moving pieces of music you've ever heard (despite the popularity of the first 40 seconds of it). People were appauled at his "crazy" style, in which it wasn't something you sat and had a cocktail party (which is what most the music was like at the time), it was music you had to sit and listen to, with your undivided attention to understand.
Back to metal, as a musician, you have to experience it first hand. I write music with growls, screams, and clean vocals, all the same, because I am trying to emulate the spectrum of emotion as I see fit. If you growl, you will feel the gut-wrenching beauty of hate, anger, desperatism, and depression. Anders hits a point close to a ghostly-gasp in some of his vocals, close to his voice cracking while screaming, and it makes chills run up my spine.
So I conclude that to think growls are the sounds of men taking shits while playing heavy music, is a very naive decision to make, and is coming from a close-minded and ignorant perspective; or at least it is to say that it's uncreative and false. I could understand saying "i don't like it", but I can't understand saying "it diminishes the music and would be better if it wasn't that way"; that's just arrogant.