(With apologies to my mother, who raised me better than this.)
My co-worker Laura often remarks that it doesn't matter if you say "dang" or "damn"-- the intent behind the word, she argues, is the same--so you might as well just use the actual swear.
And I'm not saying she's wrong because I believe that swear words are somehow intrinsically bad. On the contrary, according to the Gospel of Me, there's nothing wrong with swear words, per se. I don't believe that God plucked a handful of choice words out of each of the living languages, declaring, "These words will I make evil, that the users thereof may be hewn down and cast into the fire." Cuss words are simply words, combinations of phonemes that, by no real fault of their own, have been cast out of the upper-class lexicons; that, through the complex webbing of a language's history, have come to be viewed as a little more crass, sharp, or inappropriate than other sound combinations. Not the sort of words that ought to be brought out at nice dinner parties. Not the sort of words you'd use in a job interview. Not the type of words you'd bring home to your mother.
So while it's unfair, I'm going to go ahead and assert here that there is a real difference between saying "Oh, shooty," and "Aw, shit," even if you mean essentially the same thing.
Why? The answer lies in culture. Language is about communication; communication is dependent on interpretation; interpretation is dependent on culture. So if the culture in which you function dictates that one way of expressing something is more acceptable than another, word choice-- even when expressing the same idea--will affect the people using and hearing it in very different ways. A speaker's "core intent" (the primary purpose of the communication) is only one small component of communication; a more essential consideration is the way this intent is expressed, particularly since "peripheral intent" (other, less essential, motivations for the communication) often emerges in the subtleties of word connotation.
For example, if a lady in Wal-Mart has been standing, for the past twenty minutes, right in front of the hair dyes you want to quickly peruse, your intent in communicating with her would be to get her to step aside for a moment so that you might quickly take a glance, select the "Dark Mahogany Brown," and get the heck out of the hair care aisle. You could communicate this core intent in a couple of different ways, depending on your peripheral intentions. For example, if you wanted to display your irritation, you could tap your foot impatiently and say, "In case you haven't noticed, I'm WAITING!" Or, if you were hoping to shame the woman, you might holler, "Move your big fat ass, lady!" Or perhaps you wanted everyone involved to feel like they'd been bathed in rainbows and butterflies at the end of your interaction. If this were the case, you might politely nudge your way in and say, "Excuse me, could you please schootch over a teensy weensy little bit while I take a quick gander at the Loreal hair dye products? Thanks! Thank you! Thanks so much! Sure do 'preciatecha!" The core intent of the three previous statements was the same, but in the varying expressions of this intent, three very different messages were sent to the oblivious hair-dye row blocker.
Or, say that one of your three roommates has contracted a stomach bug and it's fallen to you to explain to the other two that the bathroom needs to be kept clear in case of emergency. Depending on your peripheral intentions, you might choose one of the following ways of expressing this:
a) "Please know, darlings, that one of our dear companions has found herself with some digestive issues and will therefore need to be given unfettered access to the rest facilities for the next several hours."
b) "Try to be as fast as you can in the bathroom! Jill may need to run in at any moment!"
c) "Jill has the shits! Stay out of the can!"
And so it is with swearing. You might be saying essentially the same thing with a swear word or a swear substitute, but you express different things depending on which one you choose. "Goldarnit," for example, might mean,"I'm in pain, I'm angry, but I choose to remain within accepted cultural conventions in expressing my pain." "Goddammit," on the other hand, could communicate something like, "I'm in pain, I'm angry about it, and I'm willing to forgo social and/or religious norms in order to fully express my outrage."
And what if your intent in communicating is purely to shock and offend all within the range of your voice? You're not going to use a swear substitute: generally the point of using a swear substitute is to keep the offensiveness down to a minimum. If you're with a group of people you don't know well, you're more likely to describe a bad day as being "crappy," rather than "shitty." However, if you are with a group of people whom you think could stand some lightening up on the prudishness, you might intentionally choose to describe your day as "shitty" and then gleefully go on to describe the "stupid bastard" in the supermarket parking lot who took up not only two but three parking spaces with his "big-ass" pickup truck, watching with delight as your audience squirms uncomfortably and flushes a little red.
Another thing to consider is that a speaker's intent in using the same words or phrases will vary, depending on the recipient of the communication. If I were to flip off a stranger who had just cut me off in heavy traffic, he would probably interpret my communicative intent as being something along the lines of, "F--- you, you jerk." And I, knowing that the stranger would probably interpret my gesture in this way, would probably mean it that way. On the other hand, when I flip off my husband, he knows that what I mean is, "I'm annoyed with you right now and know that you will find this gesture to be both slightly offensive and somewhat amusing. So by flipping you off, I am simultaneously expressing frustration and diffusing a potentially tense situation through the use of humor." I know that he knows this, so I use the gesture without the intent of communicating its more...traditional...meaning.
And of course there's the possibility that you're spending time with a group of people to whom swear words--and not substitutes-- are the accepted norm. So if you stub your toe and say, "Oh fiddlesticks!" instead of "Oh damn!" you are using your choice of language to both express discomfort and differentiate yourself from your peers, perhaps sending a message of rebellion or snobbery.
And even on a more subtle level, I would assert that there is a difference between swearing and swear substituting even when you're alone. You're all alone, you drop a glass, it shatters, and you make a linguistic choice: "Son of a gun!" you might shout-- or, if you are more linguistically rebellious, you might holler, "Son of a bitch!" As a speaker trained to recognize the difference between a "swear word" and a "non swear word," your choice of language when alone says something to yourself. It might be an almost microscopic psychological difference, but the difference is there.
So there you have it, bitches. Swearing and swear-substituting are different-- but I wouldn't say that their primary difference is a moral one. What changes the morality of a communication is the emotional/psychological impact the choice of language makes on both the speaker and the listener. But I could go on about that for another ten paragraphs, so we'll leave that discussion for another day.