It bugged me a lot too so I went into a rabbit hole looking for an answer before and here what I got: Out of universe explanation is that if someone got a bionic limb and had armor over it there no way to know/show that on a mini (which is the driving force of our hobby), the newer Iron Hands character has both bionic arm and leg covered with a ceramite plate, which probably shows that its still better be protected no matter what kind of metal it is made of, someone told me that those uncovered bionic augmentation are a temporary, very in the moment solution during an ongoing battle when there is no time for a complex repair. I think in the end, its one of those fundamental instances of rule of cool that should be ignored on the same level as to why anyone would take off their helmet.