I think that domestic production and regulation of marijuana would work to suppress demand in the U.S. for marijuana smuggled in from Mexico. There will always be marijuana from Mexican cartels available. The problem with the present system is that for the normal, casual user, there is no way to differentiate weed on the basis of its origin; nobody at the bottom end of the supply chain knows anything about the organizational structure more than one or two levels up. Thus, it is impossible for an everyday, right-minded smoker to use his purchasing power to support otherwise law-abiding domestic growers and put out of business any suppliers that engage in violence. Under system of regulation, on the other hand, the legal cannabis dispensaries would have to check out their suppliers. Under such a system, it would be possible to prevent known cartels from supplying them. As a result, those who would prefer the safe, legal, reliable option of purchasing their marijuana from a licensed dispensary to black-market purchases (i.e., everybody) would contribute nothing to the cartels, thus drying up the resources they need to fund their audacious feats of law-breaking.
To a casual user, violence in towns across the border which might be indirectly funded by his occasional weed purchases may not be the most compelling reason to get behind the movement for sensible drug legislation. After all, our government funds and participates in all kinds of violence around the world. In fact, the average smoker might contribute more to unnecessary violence through paying taxes than by purchasing weed. There are, however, potential direct consequences to the end user of using marijuana supplied by cartels. Here is why.
A Mexican drug cartel, lacking in compassion for the user across the border, may see little problem in making his product somehow heavier. This could be achieved by sneaking in either poor quality bud, other herbs altogether, or metals potentially containing toxins. Under regulation, this could never happen. Obviously any supplier who engaged in adding harmful chemicals to his bud would be blacklisted and probably imprisoned; moreover, samples of bud could be tested beforehand to ensure that harmful additives never reach anyone's lungs in the first place.