It's the most enormously frustrating thing for me in this game. The mere image of a discliplined wall of pikes behind a gate and a bunch of stupid Romans to march into it makes me all happy. When I was playing as Scipii and set up a solid box of Principes around where the enemy was battering through, I was enormously happy with my own obvious tactical brilliance rivalling Alexander the Great himself :D (worked like a dream as well, three units of Principes handed 1400 Gauls their arses back to them on a platter).
I try that with my Phalanx, which is supposed to be the best defensive formation in pre-gunpowder combat, I get mashed because they make gaps by moving to the right!