Active-Active non-floating IP requirement

In theory, I think you're right. There's no technical reason at the IP level that a non-floating IP would be required.

However you're usually using the non-floating IP's for monitoring. Without those, you couldn't monitor because the monitoring would move back & forth (Probably to the management interface which wouldn't be valid)

Yeah, I've had situations too where I think the non-floating isn't being used. But I have yet to run into an issue with having them, while you're sure to run into issues somewhere if they were optional.