Indeed, for a multitude of reasons...the least of which is pilots not being able to wrap their head around the difference between BEING PIC and LOGGING PIC.

OpSpecs aren't controlling for what a pilot puts in their logbook. 61.51 is quite clear what can be logged how under what circumstances, and the ability to log PIC per 61.51(e) differs greatly from the FAR 1 definition of Pilot in Command, which differs greatly from what OpSpecs and FOM/GOM guidance say about assigned PICs and SICs.

That being said, just because somebody *can* do something doesn't mean they should, and as a Part 91 guy, I would look quite unfavorably upon a 121 FO having turbine PIC in their logbook "because 61.51e says I can".