It's easy to spot when agile webdev types don't do this... you end up with a responsive mobile site that is missing half the features that you want.

It's easy for sharpie commandos to sort interactions by priority and build a user story that hits most of the priority interactions. Splitting isn't natural especially when the designers aren't very experienced.

IMO somebody representing the business who isn't a designer needs to be in the approval process to throw back work when all of the necessary interactions aren't captured.