First, what is a swear? I know, sounds kinda stupid, but its good to start at the basics. As I understand it, a swear is an emotional word that some people consider to be offensive. They can show extreme pain, anger, or any other intense emotion. A lot of people, myself included, are drawn to these words in those situations because they fit so nicely to what we feel we must express. And they are highly effective. But there is more to them than saying what we feel should be said. They are offensive to some people. Many songs offer a censored or edited version to explicit songs, many higher rated movies have to be edited for TV showing, and it is considered bad taste to say them in public settings.
I have talked to a fellow Christian about this, and he said that when used in the proper situations, swears can benefit what is trying to be conveyed. To discipline someone becomes much easier if the occasional swear is slipped in to show the dissatisfaction and anger felt by the discipliner. People remember when they have been cussed out for legitimate reasons. They appear to hurt and affect us more than many other words can. I have also talked to non-Christian friends who get tired and annoyed at people who are constantly inserting f-bombs in anything they say. You know them. The every-second-word-is-a-swear person. It gets tiring. Of course, almost anything used in excess gets tiring and bothersome, if not bad for you.
So what is the answer? For that, I look to the Bible. Though it never specifically mentions "do not swear," some other passages stand out to me. "Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen" (Ephesians 4:29). This passage especially sticks to me. How often is a swear used that it helps build others up? Like my Christian friend said, it can be powerful when used correctly, but I cannot recall any time where I benefitted from being swore at. I remember being hurt, shocked, and scarred from it. Those aren't beneficial.
As for being unwholesome, how many people tell their kids not to swear? Everyone knows that it's not good for little kids to be saying those things. So why is it ok for adults? The only things I have found that are allowable to adults and not to kids end up causing more problems than good, such as alcohol, gambling and "adult" entertainment. Jesus said that we should come to the kingdom of God with the innocence and excitement children do. Even adults must edit them from certain situations. A politician would not say that his new bill is going to be "f-ing perfect for the city." It portrays what he thinks of the new bill, but it is not appropriate. The same for a teacher calling a student's work "s***y" or a TV host saying the performance was "the f-ing s***." In a world that is so lax already with restrictions, is it really the best to push this one, too?
Other passages back up the one in Ephesians. Proverbs 10:31 says, "a perverse tongue will be silenced." 15:4 says, "a perverse tongue crushes the spirit." "I will keep my tongue from sin" (Ps 39:1), "You love every harmful word, you deceitful tongue!" (Ps 52:4), "no human can tame the tongue. It is a restless evil, full of deadly poison" (James 3:8), "The tongue also is a fire, a world of evil among the parts of the body" (James 3:6). Many people would argue that the tongue talked about here refers to lying and deceit and gossip rather than cursing and there are tons of passages that fully support that directly, especially in Psalms and Proverbs. Those types of speech destroy people and the results of such words are explicitly started. Yet the ones here lead to me to a different concept. Harmful words, evil, perverse...these all appear to be a bit more than those mentioned before. Our words are not to offend or hurt people, but bring hope and joy from the Kingdom of God. Does a swear do that?
Paul said that "Everything is permissible, but not everything is beneficial"(1 Cor 10:23). If a swear can be used to build others up and aim them towards God, then by all means, use it. However, I have not encountered any that have done that for me. If I utter one under my breath by accident, I immediately repent. I get uncomfortable when others I see as good people use those words. I've never heard a person say they wished they swore more, but rather that they did it less. So should a Christian swear? I have provided as unbiased position as I could with the evidence given and covered as much as I could think of. I think it all boils down to is the question, "Is what's coming out of our mouths pointing people up...or down?"