As the proverb goes, "You catch more flies with honey than vinegar." There's definitely something to be said about being cordial with others. Being perpetually nice to someone regardless of how they're treating you speaks about how you feel about yourself more than how you feel about someone putting you down. If you're confident in yourself, you're less likely to be affected by the rants of others against you. If you're less confident, it's likely that you'll feel the need to lash back.
Anyway, being nice to others (as well as yourself) should be everyone's first priority. I'm not saying that we need to all hold hands and sing Kumbaya together, but at least be civil and live by the old Kindergarten rule: If you don't have anything nice to say, don't say anything at all.
There are ways to disagree with others without telling them that they are idiots and wrong. If you can figure out how to do that, you will be able to educate and influence others who may otherwise be less informed. Although, some people just can't be taught or spoken to because they're not willing to accept any thoughts other than their own. In those cases, you just need to "Inception" their minds :)
Stay positive, be happy, be you!