The preheating of the secondary air is to prevent cooling down the volitiles so the won't burn. If you introduce cold, outside air, the volitiles will remain as soot, and wasted, unburnt energy.
The air above a fire is usually too low on oxygen to burn all the volitiles, and it takes the right combination of oxygen, heat and volitiles to create a fire. This 'right' combination often results in a 'puff-back' in handfed stoves.
With the hot blast stoves, the secondary air usually comes in through a duct that is located alongside the outside of the firepot, so the firepot is doing the preheating. The vent ring around the upper perimeter of the firepot is also very hot, so the air probably is heated to ??? 300-500* ?? just guessing, it would depend on the fire, firepot liner, airflow rate etc.
The design for the HotBlast stoves is well thought out, and I'd love to watch one light off the volitiles.. it would be an impressive sight I'm sure.
When I was burning Bit coal in my big hand fed boiler, I created a preheated air source, and when the conditions were right, the secondary fire was very impressive. and it reduced the soot buildup quite a bit.