kiai are best used when the attacker is not expecting it- so if you kiai every technique, not only will you strain your voice, but unless your kiai is very powerful, it will lose effect. It's quite useful with an atemi when unexpected (as my sensei have demonstrated on me).
Let me put it this way- what would your reaction be if a person came up and suddenly screamed in yor face? Now, what would your reaction be if you knew that person was going to do that as they approached you?
Many of the best things in life are used sparingly.