The Swayze subplot

Rating: 9/10 (6 August 2005)

This is my biggest sticking point with this movie. Was the subplot needed? I know it showed that Donnie could be dangerous and added to the mystery of Donnie's psychoses, but why was Swayze's character Jim crying at the end? Had he also been saved by Donnie's sacrifice and would now give up this wicked ways? Or was he crying for his own wickedness?
I know it added another level to this film, and showed Frank's advice to burn down the house to be based on moral values thereby showing the Frank/Donnie relationship to be based on good not evil. But I still feel Swayze was a subplot too far. What do you guys think?