I used to smoke cannabis. I'd get through about £20-worth every 2 weeks, comparable to moonster's DP. I nearly always smoked it only in the evenings, after work, although it was not uncommon for me to smoke it first thing in the morning on a weekend. I smoked like this for about 5 years, alhtough I have not smoked (either cannabis or tobacco) for 3 years now, as I gave up when trying to conceive.
During that time, because I walked the dog daily, continued to hold down a job and perform well in it, and maintained relationships with friends and family, I told myself that cannabis was no big deal and didn't affect my functioning in any way. And it didn't affect my ability to to what was necessary, I will admit. However, within a few months of stopping it, I realised that my energy levels had shot through the roof. Cannabis DID rob me of the motivation to do anything other than what was necessary. I spent my evenings pleasantly relaxing after a hard day's work (i.e. getting lightly stoned), whereas today, as a non-smoker, I find I can relax just as well in a much healthier manner that actually involves having a life.
Getting stoned reduces your motivation, has a detrimental effect on your energy (as does tobacco smoking), and tends to limit your social life as many users tend to subconsciously avoid situations/people where they cannot smoke a joint and can't fully relax in a social scene where they think their habit may be disapproved of.
I agree people can get hysterical over cannabis. Getting hold of a bit of weed is in no way like trying to get hold of crack cocaine. Cannabis is usually obtained via friends of friends. In all the years I smoked it I never had to meet with a scary drug-dealer type down a dark alley somewhere. I also find that most cannabis users are usually more agreeable and way less aggressive than your drinker types. I would far rather be friends with a regular stoner than a heavy drinker, for example, and I still have some friends who take recreational drugs. However, I don't want either habit in a partner.
Cannabis is usually smoked, and taken in this way each joint is about 5x more damaging to your lungs (BBC news report). Most cannabis smokers are habitual users. I know there will be some members on here who only ever smoke it occasionally/recreationally, and others who know people who only smoke it occasionally, but they are in a minority. Most (not all, but definitely most) users are unable to 'take it or leave it' and find that it gradually becomes a regular part of their lives. I bet that if most of you think of the people you know who smoke it regularly you will see that while they may hold down jobs/marriages etc, they don't actually do that much outside of these roles - few hobbies to speak of. That's my experience, anyway. Apart from a small minority of users, cannabis prevent people from being the best person they can be IMO. I think this is where the OP is coming from. She sees that her DP is not the realising his full potential, and she is angry because it is making her feel like she and their DDs are not enough reason for him to do that - that he values his cannabis more than them. There is also the difficulty of the role model he will present when the DC are older. Cannabis should be made legal IMO as it is in a different class to something like heroin, but while it remains illegal, it is setting a bad example to a young, impressionable child who does not have the life experience or intellectual maturity to fully understand how one illegal thing can be ok but another is not ok. There is the danger of thinking "Well if that's ok, perhaps this is too?"
FWIW, moonsters, I'm afraid that while I see where you're coming from and I'm personally in agreement with you, the posters who say you cannot change your DP are right. Nothing you can say or do will change his mindset, as only he can do that. One thing you could try, if he is open to stopping but simply can't, is to get him to read the Allen Carr book about smoking. If he replace the words nicotine/tobacco with cannabis, it may help him to see cannabis in a new light and drive him to stop.
All the best. (sorry for the very long post)