Because none of the available Reinforcements functions does quite exactly what I needed for this map.  Requirements were that 1) reinforcements could, but need not, carry cargo, 2) reinforcements could stay in the game and not be removed automatically after unloading, 3) work with both APCs and helicopters, and 4) allows for some custom action being run on the created units.

Reinforcements.Insert violates (2) and (4), and Reinforcements.Reinforce violates (1) and (3).