The way we conduct our speech is an extremely important aspect of being a Christ-like example. Next to the way we look and dress, the words that come out of our mouth are often one of the first things people judge us by. We have such an opportunity to show Christ to others with the way we talk. That doesn't mean that we're going around preaching the Gospel in every sentence, but we can certainly reflect reflect God in the things that we choose to say - and the things that we choose NOT to say.
Profane language is one of the most difficult things to guard ourselves against. It's everywhere; TV, movies, music, signs and advertisements, everyday speech among friends, etc.... In fact, most people in the world today probably don't consider the "bad words" they are saying to even be bad words. But if we surround ourselves with that kind of talking for long enough, we too will become jaded and it won't be a big deal to us. And it's not only bad language or 'curse words' that we must watch out for, it can be anything from insults to angry words to gossip. One thing that Paul says in that verse in Ephesians that really sticks with me is when he says, "... That it may impart grace to the hearers." The way we talk needs to impart grace to those who are listening. And profanity, gossip, backbiting, etc - these aren't things that do that.
James tells us in James 3:8 that the tongue is an unruly evil, full of deadly poison. We have to be careful with such a powerful tool.
Speak in a way that glorifies God.