Pain is a vitally important function of the central nervous system. Children born without the ability to feel pain (congenital insensitivity to pain and anhydrosis/HSAN IV) rarely live to puberty.

There's very little to be gained from an implant that continuously supresses pain in an otherwise healthy person. It dramatically increases the risk of dying from preventable infections or permanently damaging tissue, bones and ligaments due to overburdening, untreated lesions or fractures.

It'd basically decrease combat readiness in the long run.

Have you watched the Manchurian Candidate?

Zynically this instantly came to my mind, too.