In accordance to the thoughts here: Comprehensive Combat Revamp - #5 by Deus
Why not make this defensive tool a separate part that acts almost as a shield, blocking damage from a certain direction but leaving the rest of the cell vulnerable and slower? We need to increase cellular-player interaction diversity, and the pilus and defensive plates would be rather different in function.
Bird says we can look at cocoliths to serve this role. Thecal plates could also serve the same role.