I don't know how true this is, but I've heard some talk that certain mobs in the (are we ever going to get it?) Eureka content will count for S-rank and A-rank hunt kills. If that's true and they're farm-able in that zone, then those requirements probably make more sense in that context.

Problem is, I don't know if that's even true, how farm-able it would be even if it is, and... will we get Eureka before we die of old age?