This isn't really a question specific to Mormonism, but I'll take a stab at it. 
 
Why do "bad" words get censored on television? Why are certain words not allowed in G or PG rated movies? Are there certain things you would say in front of one group of people but not in front of your mother or someone you respect? Answer that and I think you've answered the bulk of your question. 
 
I think what makes a bad word bad is just that people consider it bad. Seriously. :) Social norms come to consider a particular word very foul, and so people who don't use foul language avoid it. 
 
Yeah, we should be curbing our bad intentions. But hearing someone use "gosh darnit" instead of something much "worse" doesn't imply they *aren't* trying to curb their bad intentions (i.e. maybe they're trying to do both). 
 
At the same time, there is value in clean speech (see, for example, Matthew 12:36 among others). I personally believe that foul language is offensive to God and his Holy Spirit, so learning to school your language is important, just as is learning to control your thoughts and actions. 
 
Outside the context of religion, crude language is a sign of immaturity and lack of education and/or self-discipline anyway. Swearing is just a dumb way to articulate your feelings, so even if it weren't considered "bad" socially, it'd still be lame. :)

We refrain from foul and vulgar language because it is offensive to God and he warns us through his Prophets that we will be held accountable for our immoral language and thoughts as well as our immoral actions.  
 
In the Book of Mormon a great prophet tells us "For our words will condemn us, yea, all our works will condemn us; we shall not be found spotless; and our thoughts will also condemn us; and in this awful state we shall not dare to look up to our God; and we would fain be glad if we could command the rocks and the mountains to fall upon us to hide us from his presence. But this cannot be; we must come forth and stand before him in his glory, and in his power, and in his might, majesty, and dominion, and acknowledge to our everlasting shame that all his judgments are just;" (Alma 12:14-15) 
 
Using foul language separates us form God because it is a sin to use such language hence Mormons do there best to never use these inappropriate words.  
 
Thank you Mike for your question 
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