But if he changes the reasoning enough that the other five differ from him significantly enough, then he's not really in the majority anymore, I think is the point. He can't change things terribly much by joining a bloc that is already a majority without him.
Actually, he can. Assuming that the liberal four back a straightforward right to state recognition of SSM, then if Kennedy alone issued a narrower moderate hero opinion, that would become the controlling opinion. But if Roberts issues a different narrower opinion, then unless the liberal four explicitly back one and not the other there would be no controlling opinion which could be used as precedent in unrelated cases.
Seems like a big assumption, that the liberal bloc would just have no preference for one opinion over the other.