I've never heard that aiki and kiai are opposites. I think where there is no kiai used, it's generally from the idea either that it's superfluous or that it is being used, but silently. O Sensei didn't make loud screams after every technique, so it follows that it's not always needed, or not always expressed loudly.
I would say from my training that it is always needed but never needs to be loud. Kiai is not about how loud you shout.