Correction again.
The rear wheels do not need to slip get power to the front wheels. But you might, unavoidably/unintentionally, encourage them to slip in order to get power to the front wheels by unnecessary acceleration. That can very easily happen on slippery roads. If what you said was true you'd have to burn rubber to get the 0-60 AWD numbers cars get from both motors. And you know that's not the case.
This is partly
@StealthP3D argument. That he feels you can instantly engage front wheels (as needed) with the prefect feathering of the throttle enough to get power to the front wheels without the rear wheels slipping. That simply is not true all the time, and not easy to do, nor should you even have to think about it. It's a no brainer to get front wheels going on dry. It's even a no brainer on wet. But it's not so easy on snow/ice and behavior does not work.
We want preemptive power going to all wheels, not wait for acceleration of any kind.