What are the benefits, if any, to the diet, physical conditioning and body-engineering typically associated with sumo? Can an individual intentionally build large amounts of fat in a way that could be considered responsible, healthy, and useful outside of this sport? What are the limits to this?
To say the diet is "healthy" would be wrong. However, it isn't as bad as it could be. Another article sites that the majority of the fat in a sumo wrestler is subcutaneous fat, and as such is less of a health risk. Quote follows:
At first glance, it looks like the two articles contradict each other. However, understand that there are differences among individuals, and some are more prone to diabetes vs. heart attack, etc. Any time you have an excess of fat, your body has to work harder to move. Adding weight too fast will cause your heart to not be able to keep up with the new demands of a heavier frame.
To answer your questions more directly:
In sumo wrestling, more mass usually means more of an advantage in the ring. The number of applications where the person having high mass being beneficial is very limited. Another place I can think of where high mass can help is with power lifting, but if you gain too much mass your body proportions will limit your overall total.
I can agree that if you are going to have the mass of a sumo wrestler, the way they go about doing it is probably the best and most responsible approach. That said, even if the amount of visceral fat is low, having large mass can be harmful in its own rite. While hardly a sumo diet, the cautionary tale of C.T. Fletcher highlights the danger of increasing your mass too much.