Hi,
If during my turn, I activate (reduce the command value) a unit with bows to better position itself to a possible enemy charge. But instead of firing, hold the action for later use as a reaction, should I reduce the command value again when doing so?
Moving into position and then holding their action= 1 Command Value
later in the game round: Reactionary activation to respond to a charge= 1 Command Value (assuming they do).
It does a couple things giving you options for events that may develop during the game round, but yes it preserves their action and therefore they are not exhausted.
lets look at a possible sequence of events.
In your example above
During your turn you move up into an elevated position and hold your action waiting for some melee squad to move forward. 1 Command Value for you.
Your opponents turn:
Your opponent knows this is a problem and does not want to lose his squad to your bowmen, and during their player turn moves a spellcaster into position to blast your bowmen to little bits.
You then decide to use 1 Command Value to interrupt gameplay with a reactionary activation to use an action ( a move action) to move your bowmen out of sight or behind better cover to help survive the upcoming spell power.
The Spellcaster now with no target simply holds his action waiting for someone else to appear and opens up the path for the melee squad to move foward. If someone else appears he may use a reactionary activation to blast them.
Now of course you could of shot at the spellcaster instead using a reactionary activation, or even decided to hold tight against the incoming power to save your action for the charging melee squad.
Holding your action really helps in giving you options for events as they unfold.
So holding the action only serves to avoid being exhausted, the command expense must be done anyway.