Raul:
I think your point that the load recommend by the manufacture is a fixed value, which is independent of the system, is indeed an oversimplification of the intent of the recommendation and cannot be sustained by any reasonable technical analysis. I agree with Nsgarch that the recommended load is simply a ball park figure. Ultimately one needs to experiment to determine the optimal load value for the particular system.
The load is system dependent. The impedance of a MC cartridge is not purely resistive, it contains a reactive component. The system in which the cartridge is used will impact the transfer function of the cartridge (flat frequency response, etc). The system here includes tone arm, turntable, phono interconnect cable and the input characteristics of the phono preamp, etc. For example, the cartridge impedance may be impacted by stray impedances due to the presence of external interferences, the interconnect has nonzero impedance and the input impedance of the phono section is not infinite thus it impacts the value of the loading impedance. In summary, one needs to experiment, using the recommended load as a guide, to determine the load value that gives the best-perceived flat response.
If you wish I can go into more technical depth.