Thats quite a debated point. There are arguments that even though it may be worse weight for weight, cigarettes tend to be smoked in greater numbers than joints. 
Weed also has positive benefits, see Neuroprotective effect of cannabidiol, a non-psychoactive component from Cannabis sativa, on beta-amyloid-induced toxicity in PC12 cells. 2004] http://www.ncbi.nlm.nih.gov/pubmed/15030397
Then there's the method of consumption, when ingested ot vaped, cannabis is significantly less harmful than tobacco.