Spike Block

Spike Blocks are blocks originally planned to be introduced instead of Sticky Pistons. They would work like pistons but push out Spikes. It was later suggested to Jeb that he make should make Sticky Pistons instead and also make a Spike Block that could be pushed by them. It is unknown if Jeb is still working on these, although it is unlikely.

Uses
Presumably, the Spike Block/Piston would have been used in traps, therefore they would probably have caused damage to mobs and the Player.