There is something to be said for blocking hundreds of punches, taking hundreds of falls, and evading hundreds of weapon swings every night in a class. Successful technique grows out of balance, timing, and awareness, which grows out of relaxation. If one lives in a tense state, one’s energy will constantly be tied up in tension. Balance, timing, and awareness are vastly improved when one can interact fluidly with one’s environment, unencumbered by tension.
That being said, you are not training precise self defense when you are practicing against a predetermined attack. Detractors of self defense training misunderstand this fact. In fact, many self defense styles don’t understand the method’s purpose either. What you are training for is attributes; attributes that are not feasible or too dangerous to practice against a live classmate. You are conditioning your body to be able to react in the moment when presented with a similar position. Should your body be conditioned to block an oncoming punch, it will react accordingly when presented with one. This assumes “good” training on the practitioner’s part. Phoning in attacks and defenses leads to ineffective technique just the same. Predetermined defenses have to have commitment from both attacker and defender to develop these attributes.
Committed attacks are necessary for developing correct technique. However committed doesn’t mean “all out” or out of control. Rather it means sincere and focused. Many times I’ll have beginners or visiting students punch at me, they will most times either be out of proper distance or they will punch where my head isn’t. To make a point I won’t move. The committed attack is aimed directly towards its target. It should require the receiver of the technique a strong need to react.
Drilling martial arts, whether it be stance, footwork, or technique, and not understanding context, is the same as trying build a house with just hammers alone; just the tools but no foundation or nails and mortar to keep it together.