You don't commit your weight to a strike in sparring, largely because you don't want to do any real injury and partially because you have been convinced by your instructors how easy it is to destroy the balance of an over-committed attacker. Being cautious from both ends, we train our attacks to be weak and don't realize it. Reps are reps. A thousand reps of weak attacks hones the skill of attacking without power.
That's only a piece, because the defenses that we learn are based on these uncommitted attacks. And they work well here, for what it is worth. But the physical and emotional difference of a truly committed attack is amazing. It will blow through many of the sparring defenses. It can freeze you and take away the subtle glides and evasions.
It's not a difficult problem, the fully committed attack. The physics are pretty simple, the stuff that works works pretty reliably. But it is so rarely addressed in training (more, I think, because it is not very safe and most people cannot summon the emotional intensity on demand) that when it happens it is an alien thing.
Many of the things that don't work in sparring are beautiful for the fully committed attack. Some of the old stuff has what you need- imprecise enough to cover a wide area, gross-motor based, closing and leaving you in good position... and some of these very same things are suicide in sparring. The x-block that Steve and Scott rightly decry is a godsend when you see a flash of steel arcing towards your belly with a frothing, angry, PCP or meth dosed freak behind it.
And this is the thing. Monkey dances, sparring, dueling are fine. But someone trying to slaughter you, whether a predator working from ambush or a domestic situation yielding to rage, will be the fully committed attack. With all of the threat's weight and speed, no holding back and with the psychological intention of introducing your insides to the cold air.
This leads to risk training. It's a little tweak for some, terrifying for others. Every so often you must practice defense in an environment where if you do it wrong, you will be injured. A full power, full speed baseball bat strike to the head. Preferably from someone with the skill to put himself in a rage mindset for a single blow and then come out of it. The old two man kata, at higher levels, had a huge element of this.
And here is the test- if uke attacks with full power, speed and rage and tori screws the defense and uke in any way is able to regain control and not hurt tori, then uke didn't have the right mindset to begin with. The training would have been 'off', valueless.
I'm not suggesting this training method, because it is not safe. I do it as tori. I will do it as uke if I know that tori completely understands that I will not pull in any way and his own survival is entirely in his hands. I am reluctant to risk a manslaughter charge for training, especially if it is for someone who may choke or not understand. The stupid and clumsy need not apply.
But there is a lesson there. Especially if you can be both, the fully committed attacker and the defender.