I'm really not sure what you're talking about. Surely you want to know when the animation is fully complete, that's what the complete callback is for. You can't rely on any portion of the animation being captured in the step callback. It's entirely possible that your duration is too small or the window loses focus, or some extremely intensive blocking operation occurs and you get a single step callback and then the animation completes. Or the animation could be stopped. You need to account for this.
Use case: You run main animation and something else depends on it's progress. Maybe it fires another animation every 10% of it's progresses. Maybe it does something else.
These don't sound like use cases. Firing another animation every 10% is going to fail in some scenarios, so you'd need to provide details of an actual example for us to understand why you'd be starting those animations and how you'd handle the various "hiccups."
"Maybe it does something else" is certainly not a use case.