I do not know a lot about MD11's and it would seem obvious no airplane should have any neutral or unstable characteristics in any axis (roll pitch or yaw), but a statically stable airplane can lose a lot in terms of controllability if it is overly stable. Fly-by-wire controls and artificial stability systems permit optimizing stability to better suit a set of requirements. Transport aircraft like the MD-11 tend to be fairly maneuverable and they handle well enough in statically stable configurations, but fighters do not and it is common to make them statically unstable in one or more axes. I wouldn't assume the FAA refuses to certify designs that are statically unstable without any augmentation, only that they are safe with augmented controls.