you can still support other causes and also support legalizing cannabis at the same time. no need to put all your time and resources into only one cause. legalizing cannabis is not unimportant as you seem to imply. it is certainly important to the thousands of nonviolent political prisoners of the war on drugs and the people who have had their rights trampled and even been killed by over zealous cops trying to bust people for a plant.
I suppose you could ask the same question of many things... Should I give my money to breast cancer research or to world vision? Should I protest for animal rights even though there are many places in the world where human suffering is inescapable? Should I pick up rubbish in the park when there is a massive water shortage in Ethiopia? I think that saying that there are "better causes to fight for" is an irrelevant and weak argument...in any case. If you want to spark a debate about your views on the legal status of Marijuana, base your argument on facts, figures, personal experiences and good solid evidence.