Thorn does do more in than Saphira in Eragon, which shows that Brom should have been removed as a character, because he interferes in Eragon and Saphira interacting with each other. But aside from interacting he:

• stays outside the city when Murtagh goes in
• flies murtagh to places
• rescues murtagh
• is just there (there's stuff at the end of the book that he does, but I don't want to spoil it)