This is from animenation in the "Ask John" archives...I like his theory:

"I may be totally off base with this theory, but I'm going to guess that this may also be rooted in Japanese martial arts tradition. Part of samurai heritage and honor was to announce one's personal name and sword technique or school of training before a duel. Examples of this are seen in both the Lone Wolf & Cub manga and in numerous classic Japanese samurai films. The point of this was to spread the fame of one's name and swordplay style, give a fair and honorable advantage to one's opponent, and show respect to an opponent. In modern anime, it's probably safe to assume that the shouting of an attack name serves a dual (no pun intended) purpose. First of all, an accompanying shout or exclamation makes a dramatic scene or attack more startling and exciting, at least in theory. Furthermore, shouting an attack name and alerting one's enemy may be considered an evolution of the samurai's honorable gesture in combat. It's possible that the presentation of the technique name in writing, especially when it fills the screen, may be a throwback and homage to the traditional samurai battle flags and the traditional Japanese elevation of calligraphy and writing to the level of martial art."