Well it would be kind of weird if Aragorn didn't go with Eowyn and then at the end this Arwen chick we never seen before just shows up out of nowhere and then he goes off with her. Also he'd seem pretty arrogant if he was all like "Yeah I'm the king, see this sword? yeah that makes me King so bow before me bitches!"
So they explained the love story between Aragorn and Arwen in the movies, they showed him as a humble guy who became king because he was needed by his people, show the reforging of the sword, etc during the movie. Because movies don't have appendices, so a lot of stuff doesn't make any sense if you don't stick it in there somewhere.
One of the first rules of making movies is "show don't tell". This is also why the star wars prequels sucked. They tell us that Obi Wan and Anakin are good friends, they tell us that Anakin has been trained to be a Jedi, they tell us that this Greivous guy is really important, but we never are actually shown these things.
Books and movies are different mediums. In a book you can go off on a tangent like Tom Bombadil for a chapter and its fine. With a movie there is a flow, and you don't want to interrupt the flow with things that don't really move the plot along or expose anything about the characters.
I'm not sure about the scouring of the Shire scene. When I heard about it being removed, I thought it was a mistake. But watching Return of the King in the theatre made me think PJ may have made the right decision on that. After Aragron was crowned king everyone was getting ready to go, satisfied with a good movie. Then when we see the boat sail to the east, once again everyone was once again starting to get their things together. But we see Sam back in the Shire and you see the audience all like "WTF? More?" And when we finally see "The End" on the screen people seemed to be relieved. Maybe if there was a scouring of the Shire it would have been like a surprise action scene at the end so having another wrapup scene would have felt more natural. But from what I could see people were a little annoyed that the movie kept going on longer when they'd already gotten what they paid to see. Most people would have rathered there be less endings rather than more.
So yeah, if PJ were 100% faithful to the books maybe the fanboys would have been happy, but 99% of the movie going audience wouldn't have liked it. They wouldn't have liked it because they just wouldn't have been good movies. PJ was as faithful as possible to the books while still making good movies from them.
And yeah sticking in some characters from LoTR in there that weren't originally in The Hobbit is a smart move. People like those characters and would be interested to see what they were doing in that time period. While reading The Hobbit I was interested in knowing more about what was going on with Gandalf and the Necromancer, so it'll be cool to see that. Similarly I was always interested in what was going on in Osgiliath in LotR, so I was glad to see that expanded on in the movies.
And even Tolkien didn't regard The Hobbit as holy. He rewrote the Riddles in the Dark chapter to make it fit in better with LotR. PJ is just making more changes to make it fit in even better, while also making the story work better as a movie.