Yamamoto had grouped up the Seireitei's Shinigami into large legions to effectively attack all at once and mix in with the Arrancar army. Thereby, effectively nullifying the Vasto of White's capability to carpet-bomb the field with whatever weapons he could form from the air due to the close proximity between enemy and ally. Of course, such a precaution was only in the case that Yamamoto failed to engage the Vasto of White in combat. In which case, grouping was still the Shinigami's best method to support each other.
