Eve of the Apocalypse: Twilight

Perhaps if you buy mercenary spawns rapidly, the 2nd/3rd purchases from a base could be reduced in number of troops. This would encourage more rationing out mercenaries to get your money's worth from them.

Also, Eldreth has a pretty awesome 2 man strategy for rushing bottom/top with mercenaries which doesn't even need Incarnation (though it never hurts).