It seems more likely to me that Roberts’ rarity lies in his awareness of the need to preserve the institution he’s been given the reins to. My sense - and I haven’t exactly seen a lot of commentary leaning this way, so what do I know - is that Obama and the left successfully kicked up a wave of anti-SCOTUS sentiment in the wake of Citizens United etc., and Roberts was keenly aware of this; when it became clear that Kennedy was going to vote with the conservatives, Roberts threw his support to the liberals because he didn’t want to deal with the fallout that would result from the Court throwing a multi-trillion dollar industry into chaos and rejecting the signal accomplishment of a beloved sitting president. Roberts has always been big on maintaining an image of impartiality and reasonableness, which is hard on a Court that includes Scalia. He wants the Court to look apolitical. I’m not entirely convinced there’s more principle beyond that.
There was a point in history when being concerned with the stability of a political institution would have been deeply conservative, because it would express a resistance to change. But in the current climate, supporting a political institution, even one you’re a part of, has started to seem weirdly moderate. After all, for all Roberts’ pro-business sentiment, that concern for stability is ultimately a vote in favor of government. That seems far more interesting to me than trying to discern some sort of coherent judicial philosophy from this.