IHL,
The issue is one of frequency of use. We flew at 96% Nr up to Vne and in various maneuvers to prove that everything was safe, but we didn't want to take the stresses of that speed and Nr as part of the component lives calculations. The failure case when N2 runs down is rare, so we didn't have to assume that below 100% Nr was a typical case. With OEI practice as a common thing, we would have had a tougher time convincing ourselves and the FAA that those occurrences were rare.
I was the Chief S-76 pilot back then, and part of the decisioning. I wrote the emergency procedures section of the manual (along with Dave Wright, who is the current S-76 Chief Pilot).