If the pilot flying the aircraft "has problems" all he has to do is remove his hood or foggles and fly visually, something that hardly requires a safety pilot to perform......
If he experiences severe spatial disorientation it might perhaps take him some time to get back on his feet (so to speak). The safety pilot can then take over for a short time in order to prevent the aircraft from entering a spin or spiral dive while the PIC becomes "fully functional" again, therefore he needs to be qualified to fly the plane. Maybe that's what the regulator had in mind, but I might be wrong. I don't even know if this might actually happen as I've had no instrument training myself, on the other hand the regulator who drafted the regs might have had even less of an idea of what flying is all about than I do, so anything is possible.
Ciao,
Dg800