The AI (MTW1 at least) doesn't seem to understand it either -- its usual response to an artillery barrage during a bridge battle is to line up its units in column formation, so that if you can hit the front unit, you'll hit every unit as the balls bounce through.

Napoleonic mod is a great one for using enfilade properly as the field guns are mobile, not fixed, so it is possible to move your cannister up to the enemy flank and blast right down the line.

Even where it's not possible to get to the flanks, I'll aim my missiles at the deepest part of the enemy formation, or shoot at the opposite flank if there's enough range, thereby getting a more oblique target (which is fine until the skirmishing starts as they can run away from the wrong units ), and also creates a crossfire if his centre advances.