I would say after playing for a bit that the rank bonus isn't as much about morale, than fighting success. Ranks seem to only have an effect on long pike units since, as CBR pointed out, you have more pikes fighting at once.

I do not think there is much of a difference in how many ranks you use, except in M2TW the wider your formation, the more men you will have fighting at once, which DOES help you win. The wider the better when fighting, except for Pikes and I guess for spears as well, but spears are too weak to tell right now. Maybe after the patch.