I can understand your argument
alatar, but I think a lot of people (or 2 that I know of
) see this and think that Boromir went to the Council and the reason he joined the Fellowship was because he needed to bring the ring back to pops. He may have been reluctant, struggling within himself (which I think can be supported from the books), but to me and my niece, we felt like it showed Boromir simply joined the Fellowship so he could get his chance at taking the Ring from Frodo. Not because he wanted to do an honorable deed and help Frodo along the way, but he was just waiting for a chance to try and take the Ring from Frodo.
Now all through FOTR I did not see this, I am more sympathetic to Jackson's Boromir than Tolkien's and I think Jackson does a very good job of staying close to Boromir's character. But just that one part in TTT EE it goes back and makes Boromir show, prior motivation before even getting to the Council, that he wanted to bring the Ring back to his father.
I love the bit of TTT EE where it shows the brotherly relationship between Boromir and Faramir, I think that is also great for Jackson to show that. It's just that one scene where Denethor sends Boromir out to bring him the Ring that gets me irked.