The purpose of the base price was to give you a default price for when smart pricing is not valid. In the past, smart pricing was only valid for four months I think. Airbnb sent out an email maybe a week or two ago saying "hey! good news! Smart Pricing now works year round!"
There have been a lot of people on here complaining about prices for special holidays in the future getting overriden - my best guess is that occurred with the roll out of the improved smart pricing.
The other thing that occurred is base price went away because it now serves no purpose. Now you just set a max and minimum and smart pricing keeps on calculating way out into the future, forever I suppose.
M