Through the movie Frodo sometimes rubs his shoulder, showing there is still pain there. Then at the end him saying, "Its been (cant remember) since weathertop. It's never acctually healed."
There are some people who are not as obbsessed like us, and they would be like, "what's weathertop?" because Weathertop in the movies is only mentioned once by Aragorn. Only the Book Fanatics would know what Gandalf was talking about. My mom was like, "What's weathertop?" and then it showed the flashback and she said, "OH!"
I think this flashback is perfect for those who are ONLY movie fans... plus it's a great scene. I love the witchking getting ready for battle too, that part is awsome!
-Cap