What is the purpose of greasing BB threads?
To prevent galvanic corrosion. When grease is appied, there is a thin film of grease that prevent direct contact between two different metal. 
To prevent water and contaminants, especially salt in the winter season, that would otherwise accelerates corrosion as discussed in (1)
This will not work with plastic or carbon fiber (+epoxy as matrix) materials as grease will reduce the material's strength over time, i.e. epoxy on carbon fibre is weakened.