My idea is as simple as just being able to produce layout sizes bigger than current 216 inch limit, for being able to layout billboards, wall graphics, etc.

The link feature and linked art boxes are able to move and crop graphics much more fluid, and also, typesetting in InDesign much more powerful than Illustrator. It's simply faster to produce a mock-up and final art in Indesign and it's easier to make corrections as well.