To answer your question on advantage vs. disadvantage, you have to consider first and foremost the use. The advantage is that you can have front wheel only high (rear nuetral), front wheel only low (rear nuetral), rear wheel only high (front nuetral), rear wheel only low (front nuetral), rear and front low, rear and front high, as well as front and rear nuetral. Manuevering with front wheel drive only can really cut down turning radius especially if you rig is locker equipped in the rear (tends to push straight). The disadvantage is that you need to remove the rails from the case and grind them (not bad as it's a one time deal), but you will need two shifters or a highly modified single shifter to operate the case (instead of a single stock shifter). Also once the rails are ground, it takes alot to undo if you ever wanted to go back. For a mild off-roader I don't know if it's worth it (that;s up to the individual), but for a hardcore rig that does hardcore wheeling, it is a great asset....
Just my .02
