[thejeff]

Because it gets harder to compare. You have to figure in the costs of buying the priests, of having more provinces being hunted and thus producing less gold. The opportunity cost of using those castle slots to buy blood hunters instead of researchers or combat mages.
I agree in general, especially with nations like Mictlan who have cheap blood hunters, the other costs are generally low.
I can see two cases where you might want to empower hunters: Where you only have expensive B1 mages, like Vanjarls. Or capital only ones - Warlocks? Then the cost of getting more blood hunters is much higher.

Or when you are low on provinces to hunt. Few higher level blood mages seem to generate less unrest/slave than more low level hunters, so you might want to concentrate.