It's probably best just to use a couple of Naginata units, rather than basing an army around them. I imagine that it would be rather effective to use them to hold specific points in a line. Stick a unit in your army, let them naturally lag behind because of their slow speed, then bring them around to react to an enemy flanking move. Use them like a rear guard, one might say. If the enemy changes his attack, you can still use the Nagi to block his flank in and screen for something with more punch to pull off a full envelopment.

Even still, it would probably be better to go with lots of Yari.