Project Prospect had a scout give his opinion on long tossing. I liked it and though I would give my opinion and experience on it.
I also have a strong feeling about long toss. I believe it is a great way to gain arm strength and to warm up. I believe in the "bullets" theory to a point but it varies from person to person. I have no way to prove or disprove this as well, but I just have personal experience.
I grew up on a farm in rural Minnesota and there was very little to do besides spend time outside. My favorite past time was throwing rocks. I would throw rocks standing on our gravel driveway and throw them at our garbage burning barrel, which was an old 50 gallon oil drum. It was about 130' from the driveway and I would try to hit it. From the same spot I could throw at a 4" wide fence post that was 60' away and several trees in our grove about 100' away. I would throw at each of these targets for hours on end. If I would just be out doing chores, I would pick up a rock and throw at it. These were small rocks, not baseballs. When I was in my mid teens, I would often try throwing these rocks at the burning barrell from an area that was 250' from it and I could get close to it frequently and throw it over some times, often without any loosening up, just randomly. I know, I abused myself.
When I finally got the chance to pitch as an 11 year old, I dominated. No one could get close to cathcing up to the velocity I had. The coach actually told me to throw slower so the other kids could get a chance. In the summer of that year I flew off my 4 wheeler and sprained my wrist but continued to pitch. I tore my rotator cuff pitching with the sprained wrist and couldn't throw all summer. I didn't have surgery, but to pass the time, I learned to throw left handed. To reinforce that it works, I could throw a ball left handed as well or better than most could with their primary throwing arm after a few years. I would play catch left handed the day after I pitched from then on to rest my arm. I never played left handed in a game though.
My "rehab" would probably make most cringe but it was not doctor supervised, that is for sure, other than the standard tubing that I got from a "trainer" that stopped by school once. I found rocks that were approximately the size of baseball that were different weights. I set up plywood and posts and built a mound 60' from the target. I would throw these stones, tennis balls, wiffle balls and baseballs at the targets to strengthen my arm. I beat the plywood to hell, but it worked.
I had severe pain for the next couple years whenever I threw much and I would apply Flex-all 454 between every inning for over a year to numb it down and take a ton of ibuprofen to tolerate the pain. It didn't help most games were played when it was 40 degrees out.
I know my arm wasn't right after that but by continuing my hobby of throwing rocks long distances and baseballs long distances when I had someone to play with, I was able to build enormous arm strength. I was 5'9", 190 LBS when I graduated high school, so lean was not a term you could use about me in any way, but I could throw a ball over the 322' left field wall from home plate. The main reason I think I was able to do this was because of throwing thousands and thousands of rocks and baseballs in my youth, most of them for very long distances. I had injuries but they were off-field and I think long tossing does increase arm strength and velocity. I averaged about 1.5 K/IP in my career from 7th grade to graduation mostly because of my fastball.
This isn't really about prospects, but it was my experience. If I would have played college ball, I would have done well in Division II, I think, but I doubt I would have ever been drafted. I had several physcial issues that would have turned off teams besides my build as a short righty, but even though it was painful, I did enjoy my career playing baseball and still miss it. It's much of the reason I still stay involved with baseball in this way.
Young guys reading this, take care of yourself. Push your limits and learn the difference between pain and injury, but take care of yourself when you are injured. Just because you aren't a gifted athlete doesn't mean you can't gain strength and compete with the others who are.