BENEFITS:
The wheel supports the full weight of the vehicle on bearing races.
You can pull the axle without jacking up the vehicle. Just unbolt the hub and pull it out.
You can drive on them in front-wheel-drive with no axles in the rear.
A semi-floater has only one set of bearings instead of two and the bearings only support *most* of the weight; the rest is supported by the axle itself. Because of this the axle must be way thicker. The bearings also don't lock the wheel in place, so if you break an axle the wheel can come right off. This is why semi-floating axles have C-clips; to keep the axles from coming out and taking the wheel with it. And remember, to get at these C-clips you'd have to drain the diff. and open it up.
Full-floaters need no c-clips because the axles can't go anywhere; they're held in by being bolted to the wheel.
In general, It's easier to work on a full-floater.