I think a ballhead is not an absolute necessity with the Gorillapod, but it helps. After you secure the Gorillapod to an object, you don't want to be messing with the legs to get the composition right. So, you play around with a tripod head.
But I use the Gorillapod mainly for my flash. So, I don't need to mount a tripod head.
But on days I want to mount my camera, I will mount a 484rc2 on it.