Lomapaseo,
I think the reason that only the fan cases are armoured is that the energy released by a liberated N2 blade is quite small compared with a fan blade. Take P&W JT9D - fan blade mass about 4.5Kg and RPM 3800 or so. That's a hell of a lot of energy in the liberated blade, not to speak of the savage out of balance forces acting to destroy the bearing system. By comparison, the N2 blades are tiny, even though they spin at around 11000 rpm they contain a small fraction of the energy of a fan blade, likewise the out of balance forces.