Some people take the cover off the sliding locking mechanism and permanently clamp it in the locked position. That way the vehicle works as a purely manual hub setup. The claim is it's more reliable that way.
In truth, the ADD system (sliding collar) is quite reliable if kept in good repair, and manual hubs require their own periodic maintenance or they get sticky and refuse to engage when you need them as well.
Personally, the only reason I see for installing manual hubs is if you have installed an automatic locker in front. In that case, you'll want to be able to disconnect both wheels/axles from the diff for normal highway driving. If you have the standard open diff in front, manual hubs don't buy you much of anything. They won't improve fuel mileage measurably, nor will they extend the life of your CV axles significantly. There's some reason to argue that the CV joints and boots last longer if continuously turning with no load than if just sitting still to set up in one orientation.
