Throughout the movie, Thorin is portrayed as being a bit un-hinged, and the belief that his father still lives is part of that portrayal; but, this piece of the movie, Thrain alive at Dol Gul Dor, shows that Thorin, may in fact, be sensing that his father still lives.
Most of the extra stuff is just dialogue that was cut out and is now back into the film; but, the segment with Thrain is long enough that it makes a difference in the film. IMHO
Interesting that the Hobbit is only one, regular length, book (300 pages?)... that was made into three full length movies.