Being pleasant costs you nothing and yields tremendous benefits. And you can still disagree with people and not be a pushover. I can have a debate with someone and not be a rude, unpleasant person.
This lie is repeated pretty often. I had several periods in my life (few months of sleep deprivation after my daughter was born, stressful periods during university, etc), when it required a huge amount of conscious effort to be nice with colleagues, not to mention dealing with assholes.
What goes around comes around, in a nice way, too. :)