And to that end, the little surprises create thinking divers.
Yes, there is a difference between harassment and surprise drills. A real world issue will always be a surprise, so practicing for the surprise is good.
On my second 150' training dive, we were on a sloping wall, and without notice, my instructor drifted down to 160' and laid motionless on the bottom. I assumed that this was another one of his drills, but without being told first, I couldn't be sure. From 150', I flashed my light back and forth over his eyes but he didn't respond. I could see that he was breathing, but our dive plan was a max depth of 150'. And at the time, that felt
very deep to me, and it was the deepest I had ever been, and I had never executed a back-up deco plan. To go down after him, I would have to go where I had never been, and do a skill (backup deco plan) for the first time. My adrenaline kicked up a bit.
We had only been down a few minutes, so I knew my gas was good. I dropped down and gave his shoulder a little shake. He turned over, took his reg out and gave me a big smile, then the OK signal. Boy was I relieved. Then he pointed at his depth gauge and gave me the "?" sign and I pointed to my backup table (+5, +10), and we finished the dive. Because of all the pre-training, it was easy.
During the de-brief, I asked him if I did the right thing, coming down after him. He said, there is no right answer to that. He said, no one can tell anyone else how much risk one should take to rescue someone else. Every situation will be different, and each diver has to asses at the moment of the problem if they should take the risk or not. He said that anyone who judges another person for the decision they make at that moment has probably never been there themselves.
The purpose for the drill was to train me to think. Skills+thinking=safe dives. If a diver doesn't train for this, when it really hits the fan, the thinking part might go out the window. It might not, but you don't know for sure because it's the first time.
Yes, the level of "surprise" drills only went up from there. There were surprise valve drills and OOG drills hanging on the wall at 200'. Once, he intentionally silted it up to zero vis, handed me an 80cf stage, and before I could clip it in went OOG on me, so I could figure out how to get him the gas and deal with (not drop) the stage. Yes, the reg was pulled from my mouth without notice more than a few times (especially if I looked like I was hyper-focused on a task and not aware of my buddy), because, chances are, that is how if could happen.
Once during a dive, I had a real issue (minor). At the 70' switch, when I started breathing from the reg, the reg free-flowed and every breath came with a mouth full of water. I choked a bit on the first breath, then got my tongue in the place where I could gurgle a breath without getting water in my lungs. Then, because of the free flow, I feathered the valve, turning it on with the inhalation, and off with the exaltation. I knew I could always go to the lost gas scenario, but I wanted to try to make it work with the problem bottle. It seemed like forever, and it was a lot of work, but finally we hit 20' and I got off that bottle. But the thing was, I never felt any stress at all when the reg gave me trouble. And we had never talked about feathering the valve - I just thought of it on the fly. Compared to all that drilling I was doing every dive, that was easy.
And that was when I realized the value of real world drilling. Or even
more than real world. A baseball batter puts extra weight on his bat while he's doing practice swings, so that when you take the weight off the bat feels light as a feather. If you practice dealing with issues that might even be more than would really happen, when the real thing happens, you will be more than ready.
My tech buddies and I still do surprise drills at depth with each other (circumstances have to be right, of course). Since we don't know if it is a drill or the real thing, the response is exactly the same either way. And now I really do know exactly how we will respond in a real emergency, and that is a huge comfort and security when we do deep dives together.
I was looking at some 5.56 rounds the other day beside some of the 7.62 stuff........they just don't make 'em like they used ta'.
Having knocked down brick walls with a 7.62, I wouldn't care to be hit by 1 no matter how good the armour was. A 5.56 just isn't in the race. If you have a brick wall you need demolished in a hurry though, I'd recommend a .50 cal. 
