the AI is too dumb to keep a formation and too dumb to properly catagorize your troops for default(EB) formations. You gotta do it yourself, but thats a good thing. Formations are unique for every army, every enemy, and every terrain.
Take a look at some ancient battles and get some ideas, it'll make the game more fun.
Alexandrian style double phalanx anyone? i find it useful once in a while, where a greek phalanx square is too defensive, and a single line is too exposed.