I am not a programmer, but I can think of issues why this could not be implemented...
1. formation while moving...
You could not move your starting formation, due to the fact that you do not usually have roads all over the place, but just one line connecting cities... than terrain...
This could be implemented for a group of naval units, but other than that, I do not think its realistic to expect it being implemented
2. Protecting ranged units...
this would require some algorithm that would place ranged units at the end of a column, or on the sides... and then you would get people complaining how their ranged unit was at the back, and they wanted to skirmish first and attack right after that....
3. Unexpected sighting - what happens if the troops spot an enemy unit along the way. Do they continue or does the entire army stop to reassess the situation?
4. Enemy ranged attacks - what happens if the troops are under attack while moving (even from an enemy they can not see like airplanes or artillery) does the entire army stop, or just the injured unit. if the later, how does that affect the movement of the army, its formation, etc...
Dunno, while grouping units for movement could provide some benefit, apart from some potential for grouping Naval units, I do not see how it could be pulled off