in think that growl vocals for the growl vocal's sake has no sense, the vocals must represent the part of the song where they are on. Love of hate a song (like or dislike) just because it has growling voice on it is IMO stupid. I think that we all like Opeth because of the "drama factor" of their songs, and the good taste on them.
A fair judgement, Face of Melinda, Credence and so on, are fantastic songs because of the feeling put on them, I don't mind if the next record has all clean vocals in it, as long as they are put with taste and emotion, just like the aforementioned.