Understandable, but if you point out that I'm not holding my fork with the correct hand, I'm going to laugh in your face. Formal or not.
But we would probably never be in a situation in which I would have to point that out, nor would I point it out; just as Karen said, pointing it out is rude. Context is everything. How many times have you been to a formal dinner? Weddings aren't formal events, btw. It all depends on context. Formal dinners typically bore me to death and I avoid them like the plague, but if I am forced to go to one, I know how to conduct myself depending on the company.
There is certain behavior that is expected of you in certain contexts, and that is, in essence, etiquette. For the same reason you don't fart and burp in public, you don't drink water out of your wine glass if you're not 5. Granted, the impact isn't as much of a big deal as someone ripping one at the dinner table, but it's still etiquette.
Like I said, context is everything. However, if you hold your fork like a 4 year old, like in a fist, that would be simply pathetic. How you conduct yourself at the dinner table reveals volumes about your history, class, upbringing, and education.
Also, when eating soup, you gently tip the spoon away from you and not towards you when eating.
Did you know that there's actually a proper etiquette for eating a hamburger? You're supposed to use both hands, eat it as fast as you can within reason, and not talk at all while eating it. It's not considered a food over which you have a discussion, therefore it's considered rude to take too long to eat your burger.