LOTR stacking is kind of important because it's a LOT easier to complete modes in most cases stacked with multiball. Cases in point:
Gandalf vs. Saruman - There is a wandering shot that you must hit at one point. If you have great ball control you can just wait for it to move somewhere you can shoot with a trapped ball and snipe it, or if you're in multiball, just spray-n-pray and you'll probably get it 95% of the time.
Kill the Wargs - It's every shot on the playfield. I think you can see how multiball would help here.
Kill the Witch King - Combo shots are much faster to do with two balls under control on opposite flippers than 1.
Gollum - You start multiball with 4 balls if you stack gollum with multiball. You have an average whole-playfield point increase of 1.5x while Gollum is active. Gollum gives you two balls to shoot ring modes with.