I'm at Gen 4 so have to use the Quad Rocket to head to 5, and I have no clue why anyone would ever use the alt-fire (zoom) mode. The rockets simply seem to travel slower, and in a larger spread. Against ground targets they smear out and maybe one rocket will hit near a grunt, and against Titans they become extremely easy to dodge.
Is that it, or is there some other subtle effect I'm not picking up on? In what situations is the alt-fire mode advantageous?