Someone said there are no stupid questions, only stupid answers. I don't agree fully with that; but come on, this poll isn't stupid at all. But I have to say that the answer is quite simple for me: Mike's growling vox are one of the key ingredients which make Opeth's mix so delicious, varied and unique. Without it one would lose a harsh element which would make the contrasts much smaller, and that's something I for one would hate to lose. Contrasts and variety are two words which always come to mind first when trying to describe Opeth's music, as is complete. Without the two first-mentioned the latter wouldn't be attainable.
Keep on growlin' in the free world!