Minecraft 1.8.8, Minecraft 15w36b, Minecraft 15w36c, Minecraft 15w36d, Minecraft 15w37a, Minecraft 15w38a
When slime blocks get moved by pistons they only set the motion of entities in their way, but don't move them.
This causes the entity to be inside the block for a short moment (see MobInsidePiston.png).
It also can cause issues when pistons move in lazy chunks (chunks where entities don't get ticked) as the entities won't move.
Therefore I think it would be better if the entities would get moved by slime blocks as well as setting the motion.