You're right, the yell is not always needed. My instructor maintains that the yell served two purposes: (1) to help the instructor gauge his students kiai (body structure, mental intent, focus, etc.) and (2) it CAN be intimidating to one's enemies. :-)
I guess my sensei's point (and by proxy, mine) is that the yell is a by product of good kiai, not good kiai by itself.