All the ADD does is keep the front driveshaft from spinning (and ring a pinion in the differential and 4wd section of the transfer case as well) when in 2wd. The CV axle shafts always spin.

Switching to manual hubs allows everything to stop spinning when the hubs are locked and in 2wd.

Running with the hubs locked and the ADD still intact is just like the stock ADD setup.

Running with the hubs locked and the ADD disabled spins everything up front all the time, but in 2wd there is just no power going to the wheels. More of a gas mileage and minor vibration/noise issue than a wear issue.