James 3:10 - "And so blessing and cursing come pouring out of the same mouth. Surely, my brothers and sisters, this is not right!" (NLT)
When we use foul language it does a lot to hurt our reputation as Christians. We need to be above reproach, and curse words, words of hatred, and gossip do little to show people the love of Jesus. We need to watch our actions and words to make sure they are reflections of God.
Does this mean we should never express anger or frustration? No. Jesus often rebuked those that did wrong, but look carefully at His words. He did not require the use of inflammatory language to get his point across. We should try our best to emulate the strong, but gentle, way Jesus spoke to those that did wrong.