IS has plenty of use in FE2. I've barely played the game, but I've figured out that IS has been borrowing ideas from this game for a long time. Monsters, moving on the overworld, branched promotions, multi-tier promotions, multiple parties, and some more I can't recall at the moment. I'm sure I'll be able to add to the list after I actually play the game.
Could've sworn it did. Eh, I suppose that double might made it look like that.