Okay. So a unit can be ranked or in skirmish, requiring a ld test to change from one to the other. I don't really see the benefit of this, and the drawback is lots of faffing around changing from one to the other. Obviously, you will move at all times in skirmish mode to avoid move penalties. I assume ranked mode gives some combat benefit, so units will want to change into that to fight. Move in skirmish, fight ranked, what are the tricky decisions you refer to? I'm still not seeing where you are going with this. Units being able to chop and change like this, it's not working for me. I prefer some units to have special roles, makes them distinct. And I would rate skirmishing as one of those special roles.
No, you probably would never switch between the two, especially as it takes a full turn to reform. The option is there, that's all.
In open formation you either get no rank bonus, or a maximum of +1, so there is the obvious benefit of fighting in close formation.
So with most units, the decision is obvious. The 'decision' units to me seem to be units of light/medium cavalry equipped for combat.... Do you give them extra manoeuvrability but sacrifice a bit of their combat potential? And archers/crossbowmen.... Huge units are worth forming up, but small/medium units of say 10-20, you might benefit from having them in loose formation... but maybe not, depends how the battle goes.
And in terms of 'special roles', this again goes hand in hand with the customizable 'from the bottom up' way of making units. You can
design units for these special roles.
You want your light skirmishers? Make sure you don't give them too much armour so they can be in open formation, equip them with appropriate weapons. You don't need to buy them a special rule. Want a flexible unit of flanking cavalry? Make sure not to equip them with too much armour, again so they can benefit from open formation if they need to, but a good unit of 10 with spears would still pack a nice punch against flanks and small enemy units when formed up.