I'd say not a lot of time in the cruise is spent IMC, and it would be a pretty useful addition. Obviously, the people who actually account for it would probably disagree.
Less useful for shorter, lower flights in Europe, perhaps.
The IRSs are reasonably accurate for cruise flying though, so if you turn off the GPS inputs before you get to an area of known spoofing you'll generally be OK. The 76 happily flew across the pond with no GPS and few issues, other than an occasion jink to get back to where it should be when radio updating kicked in again.
IIRC, there was a US military proof of concept flight done recently using magnetic field maps and sensors as the means of navigation. Pretty incredible stuff!