There is another con to the way BB installed their heater - your battery monitor can't see the current he heater is drawing, so if the heater runs a bunch then the battery monitor will get out of sync.
I have been using LiFePO4 in my camper for 4 years now in all seasons, down to single digits, and have never needed to heat the battery, even though I have a battery heater installed. I insulated the outer door of the battery box, and keep the camper above freezing with the furnace (I have food and water in the camper). With this, the battery stays comfortably warm.
If you camp in super cold conditions and don't heat the camper, then you may want a heater for the batteries. In general, I think there are better value options than BB and adding your own heater is pretty trivial.