It should be kept in moderation though
That is the key to most things, imho.

The only negative effects from Cannabis are from the act of SMOKING Cannabis
Although I agree smoking anything causes stress to your lungs, your body has an amazing ability to repair it's self, this includes your lungs. I belive it is the sedentary lifestyle that causes negative effects, not weed. I know people who can't walk or run even a few yards without gasping for breath, and yet they have never smoked anything. An object at rest tends to stay at rest.