"Boromir's stuff" is important to those movie goers who haven't read the book - to emphazize why Gandalf fears the Ring (and just in case they thought G was a wimp!) People new to the story are asking the same question...why can't the good guys use the ring.
I think some of Jackson's dialogue actually plays better than the book. Aragorn's entrance at Bree, for example, is super.
And I also like how Legolas is the one who stands up in defense of Aragorn and goes after Boromir (this role was stolen from Bilbo).
In the book Aragorn is actually a little too forward:
'The words were not the doom of Minas Tirith,' said Aragorn. `But doom and great deeds are indeed at hand. For the Sword that was Broken is the Sword of Elendil that broke beneath him when he fell. It has been treasured by his heirs when all other heirlooms were lost; for it was spoken of old among us that it should be made again when the Ring, Isildur's Bane, was found. Now you have seen the sword that you have sought, what would you ask? Do you wish for the House of Elendil to return to the Land of Gondor?'
Too fast there Strider! - He didn't even know who you were until two minutes ago, and you want to ask if he would give up the throne for you? - Jackson let Aragorn be a little more subtle, and let Legolas handle the elephant in the room that clearly needed to be mentioned. I like how it played.
I think one of the main reasons PJ kept so much of Boromir's stuff was as OT said to provide the background that non-LOTR readers would need to know, but also to provide background on Boromir's character. Every other character within the Fellowship is around for 2 more movies and will have time to continue their character development, but Boromir has to be handled in this first one. Because he dies so early on in the quest, his character "stuff" can't be conveniently moved around to later movies, so there's not as much time to "explain" him.