"If we legalize marijuana, then more people will use it, so therefore, we can't legalize it."
First of all, the degree to which use would increase with legalization is not well understood. It is now widely acknowledged that the drug war fails to achieve its purported goal. Most people are already doing what they are prone to doing: either using or not, whether or not it is legal. Most people, when asked, say they will not change their habits as a result of legalization. They just fear that everyone else will, but there is actually no real data that would support such a theory. By comparison, the Netherlands has a liberalized cannabis policy, but has half the drug use we have in the States.
Many kids use cannabis as a way to rebel against society and to get their parents' attention. Legalizing cannabis will eliminate that factor of it, and make it as uncool for kids as any other legal activity. There may be a small percentage of people who don't use cannabis only because it is illegal, but it seems that the drug war is an awful lot of expense and lives wasted in order to affect only that small a percentage of people.
To the extent that legalizing may increase overall use, so much the better. In states that have legalized medical cannabis, there has been a corresponding decrease in alcohol use and suicides. There is still no fatal dose of cannabis. Allowing people to use cannabis instead of having to turn to legal but potentially-lethal substitutes like Spice would save lives and improve public health. Countless patients who currently take opiate painkillers could get equivalent pain relief from cannabis. The number of conditions and illnesses which are helped by cannabis is vast, so, again, allowing such a valuable medicine to be accessed by all would certainly improve public health, and quality of life.
Of course, there's also the amazing potential benefit of hemp which, if used to its full potential, would completely replace our needs for oil, natural gas, wood, and cotton. It would provide an abundant and rapidly-replenishing source of food which would effectively eliminate hunger in our country and potentially the world. So, more people using "marijuana" (or its cousin, hemp) would indeed be a good thing.