"too deadly to spar" is real, but misunderstood. What I'm calling sparring is when two opponents are set on doing their best to tag/contact/score on the other opponent. This is done in pads for protection. An opponent may score a series of points in succession, but will reset to begin again. In practice, the opponent may go down, then reset. This is sporting and there's nothing wrong with this. At the end of the tournament or spar practice everybody walks out and goes home (been there, done that).
In a true self-defense only art, this kind of interaction that I'm calling sparring doesn't work. If you spar, you fail. But that does not mean that two partners don't work out with each other with very heavy physical contact. Quite the opposite. Now, there are some hocus pocus arts that don't spar or do partner workouts...that's not what I'm referring to. In Kung Fu San Soo, we refer to our partner workouts as 2x2s (two and twos). It's differentiated from sparring, because what we do does not fit the sparring description. The partner workout is without pads for starters and the physical contact is much heavier. No pads and hitting harder. The initial attack/defense is 50%-70%. That's pretty bangin'. From there the defender will bring a constant onslaught of attacks, giving no chance for a counter. The targets are contacted with heavy hands and pushed through with some force. Control is learned and required. It appears to an observer that the attacker is really hurting them. So, why too deadly then?
Deadly is not really the right word either, although it's appropriate in some applications. What they mean is that real follow through would put the partner in the hospital and likely sustain life changing injuries. We are learning in a safe training environment, so disabling might be a better word. For example: When I was in Tae Kwon Do, the chest was a target. I might kick my opponent there. What happens when you kick to the chest? The opponent moves backwards or falls. There is no such target as "chest" in Kung Fu San Soo. Chest is a generic area. We might say top of the sternum. I can't really drive a hammer fist down to the top of your sternum. I would crack it and you would go to the hospital. It's "too devastating". But I can put a pretty good thump on the top of your sternum with the fleshy part of my hammer and you will move just the same and as soon as you react, I've got another one to meet you there and so on...each strike/leverage is designed to create devastation and damage. Each move is executed like it's my last move - the move that will end the fight.
I hope this sheds light on the commonly seen and ignorant assessment "too deadly to spar."
-Sifu Ken