Just getting a single controlled Wight requires a level 8 evil Cleric (to be high enough to Rebuke it). A level 8 Factotum without a magic item (or a level 5 Factotum with the Rod) can completely destroy your army with a standard action. Same resources being used here.
First, you won't destroy the army. You will at best destroy the chain of command. Then you still have an army of angry wights thirsting for the living.
But you're right an evil cleric 8 is a waste of resources for geting that wight.
Wizard 1 with precocious aprentice(command undead), arcane thesis (command undead) and spellgifted (necromancy) can take care of it.
And to be clear, since there's a 6k item in MIC that makes you completely invisible to all undead with no save, that surgical strike is trivially easy to land. Heck, any stealther can do it as long as they can take out a single Wight quickly (I picked Factotum because Opportunistic Piety will one shot TKO the Wight as well as a few around it). Maybe not even a stealther... with the Shirt even a Barbarian could just walk past your army, find the commanders, and hack them to pieces.
Good, good, but that assumes I'm sending the commanding undead into battle. But how about if I do it like this:
-Feed criminals/prisioners of war/kobolds to your original wight, wich shall always be kept in the safest place possible.
-Order your original wight to choose one of it's wights as subcommander. All other wights are ordered to obey the subcommander.
-Then give lesser ranks to the other other wights, stablishing an actual chain of command. Each wight is ordered to answer to another wight besides the original wight.
-Stablish contigencies for if a certain rank is destroyed. If the subcommander bites it, then the nearest lieutenant takes charge, and so on.
-You've now stablished a flexible chain of command that's not based on the actual enslavement ability of the wight. Wights in the field can be given the right to give orders whitout actually being the ones that spawned the other wights. As long as the original wight is kept safe, there's no critical point in the command chain.
During a war campaign as you spawn new wights:
-Wights are ordered to take prisioners alive whenever possible to send back to the original wight.
-Wights spawned during battle shall be used as frontline shock/suicide/vanguard troops so they're destroyed quickly and can't get out of control.
-If a single wight spawns a lot of other wights, then it is "retired", ordering it's minions to obey some other wight while being removed from the frontlines to a safe place. And by safe place, I mean entombed in a stone sarcophagus several feet under.
Contigencies for keeping control of the original wight:
-Order the original wight to order all it's new spawns to blindly obey the wizard.
-Otherwise keep the original wight out of contact from all other wights, locked in some dark place with chains.
-If for some reason you fail to keep command undead up on the original wight, then no trouble. Its spawns are still ordered to obey you and it can't take it's orders back (since as you pointed out there's no telephatic link for it to take it's orders back). Keep spamming command undead every day untill you get it back under control.
Well now here's a proper wight army, all based on a lv1 wizard and a CR 3 wight. Flexible chain of command based on the enslaver wight ordering it's spawns to obey other individuals, then making sure enslaver wight isn't in the frontline, makes sure there's no critical points for the oponent to strike in battle. It grows at an exponential rate, doesn't need to eat, sleep or even breathe (amphibious assaults!) and can easily match any oponent in numbers.
Then perhaps you should come up with strategies that don't require someone capable of rebuking a 4HD undead.
See above. Lv 1 wizard will do it. Thanks for the incentive of properly fleshing this out!
Without knowing the up front costs, we really can't get into this. The simple point is that if they were as efficient as you say, the people who could make them would likely have made more of them.
Even more? There's enough warforged in Eberron that they started their own city-state! And you can find them pretty much anywhere. How many more did you want?
Try coming up with plans that don't require anybody above level 5 and you'll have army situations as portrayed in the books. Heck, most of them stuck with level 1s in general.
Just did it with a lv1 and a CR 3 monster. Undead army is still more than possible and easily the scariest one out there if you use some clever thinking.