Yet, it's super important. (read the long article or just take my word for it.) Okay, maybe not the marshmallow study as such. But still. Would you argue that willpower isn't crucial for personal success, worldly success, any kind of success?
Sure, there are tricks to help you improve your self-control. But it's not through just trying harder.
Anyway, we discriminate against people who don't have it. (I may have been thinking about this during the unfortunate Doctor Who episode "Voyage of the damned", in which there is a fat couple who really likes food. One character makes fun of them for eating all the time. He is a jerk; they are the salt of the earth.) We hate on them, and consider ourselves justified in doing so!
It seems particularly unjust to me. We judge people for lacking willpower, because they don't know the willpower-increasing tricks. That's like judging someone in America for not knowing Swahili. And we do this all the time: any time you say "that person is dumb because he was too lazy to do his homework" or "that person is fat because he eats too much and never exercises," you're judging someone for lacking willpower.
(I guess the answer here is just not to judge anyone. Increase the radius of beings who deserve your appreciation and concern to be all humans, not just all humans with enough willpower.)