In this picture you can see the early weeds sprouting to the right and the fresh look under the pigs. Since all but the top couple inches are turned over in this process, it is very good approach for maintaining the subsoil structure that is so helpful in long term soil health. Tilling down six inches is pretty damaging to the worms and other beneficial creatures in the earth.
The final benefit is the pigs dig to the last inch in the garden. Running a heavy duty tiller along a fence line is nerve racking and I've had more than a couple rough moments when the tines get caught in the wire on a tight turn. The pigs are much more thorough and I don't need to worry about fence repair or untangling the tiller tines.
The main issue with using the pigs in the garden is keeping them in too long. Once they've turned over the soil, their running around compacts the earth and you need to go back in to till it up for planting, which offsets some of the benefits. This year, we're planning to keep them on a short leash and move them out while the soil is still fluffy.