Well of course it's less harmful, but docs/therapists/legal system aren't gonna start saying "oh just smoke weed."
There are certain genetic & neurological factors that increase the predisposition to seeking out heavily intoxicating substances like alcohol/opiates/benzos/etc. Not to mention that the effects of alcohol and weed are very different, and the effect that most alcoholics are looking for (sedation, relief of anxiety, stronger intoxication) won't be provided by weed. Sure, some people become relaxed and chilled out from weed, but I know a lot of people in recovery from alcohol and opiates who don't like it - they say it makes them nervous/paranoid/overly introspective/uncomfortable. Alcohol and other strong depressants have potent "numbing" effects on the mind.