have some imagination!
Captured troops can be tied to vehicles and missiles, or launched out of catapults. They can be used to clear minefields, shield your troops from enemy fire, or "donate" body parts to minifigs on your team who happen to be missing some. That's just off the top of my head, but I'm sure you can find a million uses for disposable living bodies on a battlefield.
If you captured an enemy, would you really just hold him there waiting for him to escape?
I said "no" to drugs, but they just wouldn't listen...