stealth_nsk
Deity
I think the reasons are a bit simpler:I don't know if there is evidence that the Mississippians used fire arrows against their enemies en masse as a military tactic, but I think there is also a gameplay reason as to why there is such a unit with a strong burning ability.
The Mississippian civilization seems like a counter to the Maya civilization. If you look at the Maya civilization, they appear to be very difficult to reach and fight with another civilization if the Maya cities are surrounded by vegetation. That won't be the case if you play as the Mississippians. You will burn their traps and units before you even reach their cities, nullifying the damage your army would have received otherwise.
1. Firaxis need more unit diversity as with new age and uniqueness system, there are too many unique units replacing the same base unit. They need something to distinguish. We already have 3 unique ranged units for antiquity and they have to do something interesting.
2. Since we know very little about this civilization, FXS allowed themselves to go a bit wilder.