Perma -
You raise some interesting points there. If, as you say, Fresnillo/Penoles does not want to loose the expertise of Dr. Megaw et. al., it is possible (though I have to admit unlikely in the realm of business), that they merely acquired this stake in order to have some exposure to Batopilas and Cinco de Mayo and other areas Megaw may have in his sites.
A further advantage of this position is that their large holdings would certainly provide them with a seat on the board and serve as a strong dis-incentive to any other potential suitors.