[PvK]

Particularly for Mictlan a high dominion could be useful, since it reduces your need to use blood slaves to keep your dominion around.

Otherwise, it depends on what your dominion scales are like, and what your neighbors' scales are like, and what magic etc. you want to use that is affected by dominion. There are many many examples of effects of this - keeping harmful scale effects away, keeping the heat where your men like it (important for Mictlan, which prefers +1 heat), keeping miasmatic dominion away, determining where various global spells take effect (e.g. Gift of Health, Wrath of God, etc.), etc etc etc. Heaps o' meaningful side-effects of dominion strength.

PvK