Although I train with belief that the warrior athlete is forged in the fire of adversity, I agree that there is no value in purposefully injuring yourself. It is paramount that you train smart. If you've never run with a weight vest or rucksack, don't start out by packing everything you own into the biggest pack you have. Be smart and start with a camel back. Later you can increase the weight to a pack with a bag of rice and a pillow. I don't generally like to use weight above 35lbs for ruck running.
Note also that when you do a ruck run, you don't run like you're doing a 400 meter sprint. Often it's more like the airborne shuffle (a little faster than a walk but slower than a run).
I design the Full Mission Profile
for the fit and the experienced. Like the WODs found on the main page, if you need to scale down, do it. You wouldn't jump into a 135lbs snatch WOD if you've just learned the basics of the snatch by watching Coach B's videos (excellent as they are).
Running with a weight vest or rucksack, at least in my workouts, is not solely for the purpose of eliciting a physical response, but rather to cause the athlete to feel the adverse effects of the additional weight. For those that I train personally, I want to create a hunger for adversity, but not at the expense of forward progression in training. I've found that once an athlete gets to the point that they are comfortable in being uncomfortable, their confidence begins to grow. This is not done in one session though.
I use a Cycle of Training
, which includes some weighted running. It is used sparingly though, at most once a week during the Extreme Training phase in the Full Mission Profiles. They are a way to culminate the weeks training in a test of physical and mental toughness.
Finally, I respect the opinions of others that speak from experience. As a mentor I take great pride in watching my trainees progress uninjured. There are some that I don't have under weight because they are not ready (and may never be). The tough part about the internet is that anyone can see a workout and try it, even though some might not be physically ready. Fortunately in this community, at least, the expectation is that individuals accept responsibility for themselves. Moral to the story here is "Know Thyself".