Lets start at the beginning. I, like most photographers who are portrait photographers, didn't go to school to learn photography. We learned by taking pictures and if we wanted to do it for a living we almost always had to start out by doing weddings. Weddings are a great cashflow. Our proficiency usually was reflected in our prices. Some of us took seminars and courses to learn the craft and to upgrade our skills. Back in the seventies we usually depended on weddings to stay in business. The good ones eventually did fewer and fewer weddings, charged more and ended up doing more families and portraits, and the poorer ones had to live off it.
During this time a guy by the name of Rocky Gunn perfected the concept of a photographer only shooting weddings. He'd book four or five on a Saturday, then send lots of shooters out to do the candids and then he'd show up and do the good stuff.
This caught on and there were many imitators.
So at any given wedding there were basically amateurs shooting under the name of a professional studio. And obviously some were better than others. The good ones did the high prices weddings and the others did the lower stuff,
it was all about money.
Later I guy by the name of, I think Dennis Regie (sp?) came along and made popular the photojournalist approach to weddings. Less formal. Now he has plenty of imitators. This allowed people without any training to blast away and consider it photojournalism.
So back to the point. Many people shooting weddings are new at it and if you aren't paying a lot of money you're probably getting them for your wedding. So for you, the guest, to show up and take the occasional superior picture, is no great achievement. But remember they have a list of what is needed by the B&G and you have the luxury of taking a few shots that you consider interesting. You have a distinct advantage.
As to ethics, I think you do what you want, take your shots, stay out the way and everybody will be happy.
I remember times people like aunts or relatives would continually get in the way then at the reception would ask me how to get the film out of their cameras. They screw up my stuff and didn't have the knowledge how to use their cameras. Others would continually fire off my slave strobes with their flash. It wasn't a huge problem but it was a distraction. The things photographers hate is when you are posing people and amateurs take pictures of that. Then when the B&G are ordering prints they don't buy the extras for some relatives because they stole your shots. Not as good but good enough for the relative.
We used to call shooting weddings "combat photography" and I'm glad I don't do it anymore. It was fun but a hell of a lot of work.
Anyway that is my rambling opinion.