[Illuminated One]

I do not think that using a decoy army to force the enemy to use gems is an exploit. It is used in reality (canvas tanks anyone?). In fact I'd love it if the illusion spells just worked for that purpose.
What doesn't make sense is archer (especially when set to guard rear commander) and mage decoys (well, the latter do to some extend, but it can get ridiculous).
Blocking an army is an exploit imo, but in that sense also cutting of retreat routes is when the thing that cut of the retreat is not significantly stronger than the disorganized fleeing army.