"What advantage is gained by losing the choppers in favor of grinders?"
Don't know, but having lived on a ranch, I would suppose it would be to eat grass or grain or something similarly reedy --- cows, et al, initially "pull" the grass up with their front "grinders" --- a hard and painful feat to do with sharp front teeth --- as anyone who has (like me) stupidly stripped insulation off of copper wire with their teeth can attest to.
OK, lets try this one. What advantage is gained by moving on the food chain from predator to prey, your cows not withstanding. :-}