Why was it decided that feigning of civilian, non-combatant status is bad? because it led to death of civilians who had no part in the fight; pretending to be your enemy's civilians bring no such issue. Although assassinating a patient is also not kosher it less relevant to the discussion about use of uniforms.
> pretending to be your enemy's civilians bring no such issue
Could you clarify where in the Geneva Conventions this very important exemption is stated?
> Why was it decided that feigning of civilian, non-combatant status is bad?
Because people start shooting civilians thinking they're infiltrators, and even enemy civilians are protected persons.