Hm, first time I've heard that suggestion.

Neither ep3 nor ep5 have "scrolling" intros, or use that theme for the intro, because I didn't feel like it was necessary in either of them. Also, those episodes have plenty of scrolling text in other levels. I chose to use battleships at the start because I think it's a better theme for levels in a spaceship.

Basically, I used those intros when there was information the player needed to know that happened between the episodes (or before, in the case of ep1), or I wanted to give some backstory.

ep2->ep3 and ep4->ep5 just has you flying in a ship from one place to another. Nothing happens to you in that time, and so there is nothing to be explained.