From what I understand the AWD system doesn't necessary wait until slip to engage the rear wheels. It is definitely front wheel bias, but even without slip, I could be wrong but I believe it's torque management, meaning on hard acceleration even without slip the rear wheels will power up to 50%, probably to help torque steer.
Still IMHO it would benefit take off to have have the weight shift to the rear minimized as much as possible. I look at it like if you have 50/50 power to front and rear wheels on take off, (100% overall) but are spinning the front a little, you just lost some of that front 50% power meaning your not getting all 100% overall potential on the ground.