smoking will wear you out vocally...it's only a quick fix for a raspy sound and wouldn't help your clean vocal abilities and would eventually probably make growling harder.
better would be to develop a good technique for growling that doesn't cause undue friction and strain on the vocal chords.
mikael must have worked this out pretty well otherwise his clean voice would be less smooth and the switching around live would not be as good. (even if he does smoke, for the sort of demand he has with touring, recording, etc, he'd only be able to do this by finding a teachnique that spared the chords)
as a singer, i would love to know how he does it... but i can imagine it would be difficult to describe and that different people with different physiologies might use slightly different methods.
i studied with a woman who worked with bob mcferrin's voice teacher/coach and she knew lots of breathing and sound production "tricks" but alas, i have no recollection of her name!