I don't know what exactly the corn starch does in a bath bomb....but I'll bet it does help cut that greasy feeling left on your skin from the OO....I think even just the SOUND of cornstarch added to a bath bomb sounds wonderfully soothing....
I'm getting ready to make my first bath bombs (well, not exactly bombs, I don't have big round molds, mine will be smaller, 2-3oz. individual soap molded ones) and the recipe I decided to use ( costalscents version on youtube) does not call for cornstarch, instead it calls for koalin clay. That also sound nice to me.
I'm going to try these in the morning, then I want to make what I refer to as a 'bath OIL bomb, or fizzy". Just a smidge more oils/less dry ingredients....don't know how it'll stay together though....