Honger,
If I had the standard OEM hub flanges(Live Hubs) then ADD allows for the front axle to be disconnected...or at least half of it. <img src="/forums/images/graemlins/zombie.gif" alt="" /> However, I have installed Aisin manual hubs that allow the axles to be disconnected at the hub making ADD unnecessary.

With the shift-fork locked(ADD-defeated), both front half axles are connected and would always be spinning when the truck is moving. The Aisin hubs simply replace the ADD fuctionality although completely versus halfway.

Running with the manual hubs locked and ADD-defeated wouldn't be as bad as running 4WD on dry pavement but it would be additional wear and tear on the front drivetrain as well as a draw on mpg.

When someone says shift-on-the-fly with reference to my truck they are assuming my Aisins are set to the "locked" position. Otherwise I have to get out and lock them.

If our trucks came from the factory with Aisin hubs then we wouldn't have needed ADD. Hopefully this makes sense. Brew <img src="/forums/images/graemlins/cheers.gif" alt="" />

p.s. - Here's a link to an ADD system document on another site.