I found this in NEC's FAQ:
Q: What would happen if illegal Firmware copies were used?
A: Optical drives are high-precision products. Usage environment and dispersion of parts characteristics will dramatically change performance. To absorb these dispersions and maintain the best performance, unique parameters are set up on every drive at each stage of production. Using illegal F/W copies may change these parameters. In such case, writing and reading performance could deteriorate, even though your drive works normally. We at NEC strongly urge you not to use such illegal copies.
Now, what the hell do they mean by that? From what they say I'd draw conclusion that every batch has different firmware to compensate for different parts, but that would mean that flashing each new official firmware release that difference would be nullified!
More likely it seems they refer to the reserved portions of flash which contains such parameters. But, flashing with "illegal" firmwares does not mess with those memory regions, so it is effectively the same as flashing the official releases.