I personally think etiquette is even more important in this day and age. People are so quick to be offended by the most ridiculous things, and I think having a "guide to proper behavior" is invaluable. If everyone knew and followed traditional etiquette I think things would be much more pleasant much of the time.
Now among your close friends and family I think it's silly to be too worried about formal etiquette, but when issuing invitations to a diverse group of people I still think it's better to try to follow it. With the "No gift" situation, you don't need to write that on the invitation for the people you are closest to - you likely talk to them often enough that you could mention it to them. If you aren't close enough to someone for the gift thing to come up in conversation, then I personally think you aren't close enough to forego proper etiquette in your dealings with them. In that case I think it's even more important to be "etiquettely correct" with the invitations. For example - I'm close enough to my sister that even though it's technically rude for me to try to control what gifts she does (or doesn't) get for my son, I don't mind calling to tell her not to get the video games I already got for him. On the other hand I don't have that relationship with his classmates' moms, so I wouldn't call them about it, nor would I put gift info on the invitations.
As for the wrong fork thing, I agree that if someone is offended if I use the wrong fork then that's their problem. On the other hand, that's the sort of thing that I think preteens and teens really need to learn. Not so they don't offend others, but so they will not feel awkward or confused when faced with a fancy dinner and lots of utensils. No one wants to feel self conscious because they don't know what to do in a situation where everyone else does seem to know, and a good knowledge of etiquette can help someone to feel more comfortable and confident in lots of different situations.