Now this is not to say that this custom is the "right" way to do things. It is just the way we do. That is the problem with these sorts of discussions on an internet forum. What is acceptable among one group of friends may be totally unacceptable among another.
So if the invitation says "no gifts," in some places that means take absolutely nothing at all, and in other places with other groups it mean don't take a gift, but a bottle of wine or flowers is okay if you want, and in yet another group it means no gifts, but flowers and wine are still assumed.
You can begin to see why strict rules about manners were developed. If there are strict rules that everyone in society follows, there is never any questions about what to do. I remember ready about English society during the Victorian era. Now they had RULES. Serious rules for those considered part of "society." Of course, there were lots of problems with that too so I'm pretty sure we don't want to go back to that kind of structure!