One side says that if you are fully committed to the attack, then any movement toward a block or a guard weakens your resolve, your power, or your intent. The other side of the argument says that this strike and block idea is exactly what the good fighters strive for.
The arts such as Wing Chun, Jeet Kun Do, Kali / Silat that utilize the simultaneous defense / offense principle of the strike and deflection being at the same moment if not the same movement are not generally oriented toward maximum physical power but rather on precise targeting. These strikes are usually finger strikes directed to the eyes or throat designed to create either dysfunction or to crush the guard of the attacker. It is the same thing in Aikido. These strikes are seldom used as an end in themselves but rather as components of another technique. Power is secondary to speed and accuracy and smooth transition into the technique being attempted. The strike can't be a separate piece but is rather an integral part of the movement.
This issue of power comes up a lot in Japanese martial arts. The whole Japanese martial concept is related to their ideal of swordsmanship. One cut is supposed to be the ideal in winning against an opponent. That ideal permeates Japanese martial arts. If you look at Shotokan Karate for example, its ideal is one blow, one death. Even Aikido, which is very flowery in practice, has the ideal of Katsu hayabi or "instant victory". The fight should be over the instant we touch (before that if the opponent recognizes what is going on). That is all from sword.
Aikido attacks are completely based on this concept. Practice trains you to deal with the totally commited attack that is designed to finish you off in one shot. The problem here is that trained martial artisits don't generally attack that way. They keep a bit in reserve until they are sure they have the opening for the finishing blow. Only when they are sure that they can finish the fight with that definitive blow will they commit 100%. So Aikido people are often not prepared for the types of attacks they get from people who train in other arts.