My understanding on the notorious Subie trannies is that the length of the main shaft is actually relatively long and when you run a lot of torque through the box, it actually causes the mainshaft to deflect/bend so that the gear teeth can begin to pull apart.
If you torque the tranny too hard the teeth will actually come apart and this is when the teeth shearing starts. If the Legacy's shaft is harder (read...stronger) it will be less prone to bending therefore less tendancy to deflect and therefore a better choice even with the softer teeth.