I think Lithwick has this pretty much right. In the Supreme Court, the narrowest opinion that reaches the same result as the majority is the controlling opinion of the Court. That makes the last Justice to join a 5-4 majority enormously influential, but only to the extent that that Justice either (i) writes a narrower opinion than the four Justice plurality he or she joins, or (ii) can persuade, by force of argument or political maneuvering, the four justice plurality to move its opinion fractionally closer to the 5th Justice's views.

Real, long-term power goes not just to any 5th Justice (if we're to concede Professor Jeffries's point that "swing justice" has a pejorative connotation) but to the 5th Justice who can move a plurality in a particular direction. A narrow, one Justice controlling opinion, is sufficient to decide a case, but it is a concession that an overwhelming eight Justices out of nine disagreed. If a Justice consistently loses intellectual arguments by a vote of 8-1, even while she wins a case, that Justice can hardly be very surprised that her views were ultimately disfavored when considered alongside cases that expressed much more convincingly, at least by vote counts, a particular set of ideas.

Justice Kennedy ought to be mindful of this risk of having his views marginalized, in the long term, even if he enjoys influence disproportionate to his single vote in the short term.

Agree completely. Patterico cites an interesting, and critical, article in his blog about Justice Kennedy's method of arriving at a decision.