Usually, this is argued as a question of symbolism; as the Massachusetts SJC said in its advisory opinion rejecting civil unions, giving same sex couples a different name for the same bundle of legal rights implies second-class citizenship.
But, as Joanna Grossman reminds us, there are times when the label can make a difference. It's only one part of her lengthy (and worth reading) column on a fascinating case making its way through the New York courts. A friend of mine is representing the gay plaintiff who is suing for the wrongful death of his partner--with whom he entered into a civil union in Vermont--and the intermediate appellate court just dismissed his suit by a 3-2 vote. Although there are lots of legal issues in the case, the basic question is whether the surviving partner is the "spouse" of the deceased person for purposes of New York's wrongful death statute. Professor Grossman says there's one argument that she could see being legitimately used to defeat the claim: that an out-of-state marriage might be said to confer the status of "spouse," but a civil union might not, under the long-developed rules of comity. Grossman doesn't buy the argument in the end, and I don't either, but she does give it more respect than she does the arguments that the court's majority actually used.
This case is interesting on many levels. Conceivably, New York's highest court could rule on the issue that's been hanging out there for nearly a decade: is DOMA constitutional? (That is, the part of DOMA that says states don't have to give full faith and credit to same-sex marriages from other states). My guess is they won't have to get to that issue; even though the high court is composed entirely of GOP governors' appointees, they are not particularly conservative on social issues. I think they'll find (as the trial judge did) that New York has no public policy that would override the usual rule of giving comity to sister states' legal acts, at least for purposes of the wrongful death statute.
But, whatever happens, it will be interesting. As I remember the procedural law from my bar exam days, the fact that the appellate court decision was 3-2 means that the Court of Appeals--the highest court in the state--must automatically review the case if the plaintiff appeals. And even if appeal isn't automatic, I'd expect the high court to agree to review the case anyway. So toss this in the hopper with the case challenging the same-sex marriage ban that's headed to New Jersey's high court among the upcoming blockbusters of 2006.