The transparency in CW1 was very helpful so you could see the underlying terrain and get a sense of thickness. In CW2 it is less important since the absolute shade of the Creeper is an indicator of density. It would also only really matter when creeper is above the surface (where this is an image background). Below ground, transparency would just show through to the dark gray background and make the creeper appear dimmer.
The other reason to not do it (the real reason), is the game gets much better performance when blitting opaque creeper than transparent creeper.