Did somebody say "manual hubs"
I agree with just about everything trail whale said, except for the lazyness of hubs. Manufacturers switched because of cost and maintenance, pure and simple. Unit bearings are cheap, easy to install durring manufactur, require no maintenance and are actually pretty strong (enough for the majority of owners). Hubs have more parts, take longer to assemble, and require periodic maintenance. That said I love my manual hubs!
Funny thing is they have been so popular with people I know I am getting ready to install two more sets. One set on my dads '05 3500 and one set on my Dealer Techs '08 2500. Looks like I am getting popular!