Where the Heart of Anti-Shurtugal Rises Again.

Well yeah, there's no real build up for Murtagh (Eragon does have one prophetic dream about him but that's so hamfisted that I can't in good fate call it foreshadowing), but Murtagh is at least still an important character in the series and his actions at the end of the book have consequences and changes the direction of the story (well, as much as it can be changed for this series. Thinking it over, removing Murtagh would change surprisingly little). You're right about Barst though, he existed purely to give Roran something to do and is almost worse than Varaug as building up Barst would have been incredibly easy. Just four or five single line references to him throughout the last two books would have done the trick. The fact that he is never mentioned even once in Brisinger is further proof Paolini only had the broadstrokes of Inheritance in mind when writing Brisinger, even though they were meant to be the same book.