Sparring, what is its value?

Sparring is not combat or even fighting. It has value but the extent of said value toward self-defense depends a lot on the context and intent of the sparring model. I do believe that sparring, as it exists currently, is more a dueling model, a social type fighting means of practice and training. Much like I expressed toward certain two person predefined drills. 

Sparring consists of a different emotional involvement. Both parties are willing participants. The model allows for getting a feel for the others fighting skills. In a lot of cases both parties are fairly equal in skill and ability. What happens if one or the other actually take it outside the dueling model and take it to some extreme fast and chaotic model that would test the others resolve and ability to handle it, both mentally and physically? What would happen if the role of tori were to not follow any protocol and that the tori would be bigger, stronger, and more skilled? What would happen if suddenly, without warning, you had the biggest, toughest and most skilled person in training attack someone outside of any context of sparring, etc.? 

Sparring is a great essential exercise to get novices outside their comfort zones. It can be used in a manner to stress the mind while stressing the body as well by taking it to a level of shugyo or austere type training, i.e. where the participants go until they drip and drop exhausted, spent and pretty much in a daze? Is this even possible?

Remember that the techniques necessary to defend against violence may not be as applicable during sparring, and vice versa. 

Bibliography:

Pearlman, Steven J. "The Book of Martial Power." Overlook Press. N.Y. 2006.