One of the goals of long-slow distance training (below 70% max HR) is that the body utilizes primarily fats as an energy wource. Fats are converted to usable energy in the mitochondria of the cells. When an athlete trains within the fat-burning zone, more mitochindria are produced. Having more motichondria is good for performance. Physiologically, this is one fo the many adaptations an athelte's body makes as a result of training.
To go for a ride and bonk, then keep riding is a rather drastic and uncomfortable way to ensure that your body is producing energy from fats. There are dangers associated with riding in a severly hypoglycemic (bonked) state. Decision making is not very good when bonked, reaction time is increased, and recovery time is increased.
It is wise to go for long slow rides to ensure that you are producing energy via fat anabolism rather than riding youself into the ground, then riding some more.