It was interesting for me to read your question because it absolutely never occurred to me that there would be parties where the child didn't open his gifts. To be honest, my first reaction is that for the children that do bring gifts it seems a bit rude not to give the gift-giver the joy of seeing their gift opened.
My children are always so excited to pick out the perfect gift for their friend! We do not have extra financially, so we have to make do sometimes, but they still always go with a gift that they feel good about. I tell them ahead of time that theirs probably won't be the biggest or the "best" but that they took time to give something nice and that will mean a lot to their friend.
Families have a choice whether or not to take a gift to a party (things can even be found at the dollar store), and sometimes it simply doesn't work out to take one. That said, it is up to the parents to take care of any issues. There was a time when a special gift had been purchased online for one of my son's friends and it did not arrive in time for the party. My son wasn't able to take a gift (maybe a hotwheel or something) and he knew that there was going to be gifts opened and was fine with that.
Bottom line - unless the invitation says "No Gifts" everyone attending expects there to be gift giving and opening at the party!