Fur is about the only fiber in a snakes diet. I dont know if it applies to snakes. but humans who get too little fiber in their diets have a much higher chance of getting cancer in their digestive systems. Snakes fed only hunks of meat may seem to be doing well but might actually have increased health risks such as this. It would take a well run study to find out for sure. I've also heard that the "gut load" has a significant nutritional impact. Snakes dont eat any veggies except in the bellies of rats.
One thing is sure though...many snakes have evolved to eat whole mammals as their primary diet. I wouldn't just switch them off of that without expecting that it carries some additonal risks.
1 adult bull snake: "Dozer"; 1.1 juvenile bull snakes: Oscar and Phoebe; 3 baby red-sided garters; 1.1 macklot's pythons