Dont get me wrong im not saying only Iwama people use KIAI or only people who dont Kiai cant be good at AIKIDO or BUDO. But from my personal experience and understanding of KIAI from what I learnt in Iwama, Correct use of KIAI is very important.
I've been on the receiving end of very powerful throws, delivered with and without audible kiai.
I've less often been on the receiving end of devastating throws I "never felt coming." they were always without audible kiai.
Now YMMV but I'm a little middle aged woman and much more interested in learning the latter.
Of course I'm also from Brooklyn. First time I was ever asked to kiai was about two yrs into training, visiting Ki Society of Virginia and George Simcox Sensei asked us to kiai while moving forward across the mat while a couple of people tried to hold us back. He said any kiai was fine. So I stood tall, simultaneously raised an arm aloft and hollered "TAXI!"
Very effective. Everybody in the dojo stood still in shock then fell down laughing.
Where I train now, folks are expected to kiai out the count during weapons kata. In all seriousness, I do have trouble with this. Just as I cannot carry on a conversation while I am painting, or listen to music with lyrics while I am reading, I cannot have my body carry out complex movement patterns with any accuracy if I am shouting.