In Slate, Doug Kmiec criticizes the Court's decision in Philip Morris v. Williams for its lack of unanimity and argues, echoing the fashionable arguments of Chief Justice Roberts, that unamimity helps promote "clear rules" because judges must "work out their disagreements before they write their opinions."
I've previously suggested (here) that this is backwards. Unanimous decisions are, on balance, likely to be less clear than 5-4 decisions:
It's not clear that Roberts’ prediction (that consensus on the Court yields clarity, precision, and narrowness) is right. Consensus-building in Congress, another multi-member voting body, is purchased at the price of legal fuzziness. The more amorphous and open-ended the statute — the more the statute defers tough questions — the more members of Congress agree to add their names to it.
While consensus building on the Supreme Court is a simpler prospect, there’s no reason to think the same basic dynamic won’t apply here too: Supreme Court justices will purchase broad agreement at the price of clarity, harming the rule of law.
In a very good post, Ilya Somin makes a similar point, writing in response to Kmiec, here:
Many of the complex balancing tests and complicated exceptions to rules that legal commentators like to make fun of in Supreme Court opinions are the result of the need to "count to five" — corral the five votes needed to create a binding Supreme Court decisons. Counting to nine is usually likely to require more compromise — and thus more complicated balancing tests and exceptions — than counting to five.