[Mushiro]

The best way I can explain this is by comparing Shinigami to Samurai's. Samurai's have a code of ethical honor that they abide by called Bushido. The rules of Bushido are very clear, and can be wrapped in a nutshell of "Be an honorable warrior". There were many techniques that were proven to be extremely effective in combat, however, some are dishonorable techniques. For example; a ninja is not restricted to the rules of Bushido. Attacking an unarmed, injured, or sleeping man is considered fair game to a Ninja, however a Samurai would never attack an unarmed man, or even someone they know is far weaker than themselves. It's far more effective to win a battle by attacking an unarmed man when you have a sword, however doing so is without honor, and if you were a Samurai, you will have broken the laws of Bushido, and be considered a dishonorable warrior.

So, perhaps there is some code of conduct in place in Soul Society that forbids Shinigami to use certain techniques, even though they're highly effective. Using Danku might save your ass, but there are other techniques to blocking an attack available, but they may not be as effective. Byakuya knew it was kind of cheap to use such a technique, but his opponent was playing extremely dirty, which warranted Byakuya to use it. Since he was in Hueco Mundo and not Soul Society, he was able to use Danku. Had he been in Soul Society, he would probably have been arrested.