From: South Carolina, USA
The complications involved in replacing a unit in the line, with one of an ally, are made much more complicated when the unit in line moves second, and is followed by the enemy phase. This complication can require you to leave the unit in the front line, and this is realistic. In fact it could be modeled more carefully in war games. The disengagement of engaged unis for replacement is a very difficult action. The fact that you have to consider this, and all the complications of Alliance are indeed a good feature. Certainly keeping national forces together is critical, for morale reasons, not to mention supplies, especially ammo.
I agree this is one of the really neat features.