It could be argued that a vocalized Kiai could be considered the equivalent to calling out for help (though I doubt the effectiveness of that ploy). IMO it's more likely to alert an aggressors compatriots than to elicit any assistance from strangers.
I've also heard it stated that there is an "intimidation" factor that presented with the use of a (loudly) vocalized Kiai. Personally, I find this to be one of the most ludicrous of the stated reasons. It reminds me of the "intimidation" argument presented by Police departments for their implementation of the PR-24 (side-handle baton). If someone isn't (already) "intimidated" by the firearm at your side, why would they be intimidated by a stick with a handle on it's side?
The same is true with the use of Kiai, yelling loudly while attempting to hit someone else does nothing to intimidate the perpetrator of a crime (the aggressor). They expect someone to yell (to some extent). Yelling loudly (yourself) could be considered motivation for them, to shut you up (?).