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. :)