Starting to sound like an anarchist....
If nothing else, documentation. Birth, death and marriage records have for many been the only thing to document that they were there. Ask any genealogist or historian how relevant those records are. Not to mention it ain't bad to have the option of going to the justice of the peace if you're not the churchgoing type.
Beyond that, really I don't think the government should define WHO can get married. The EPC does apply, though not directly, since the Constitution doesn't mention marriage it's defined by the states or the people where the states don't, arguably it can still regulate the manner in which it's defined.
As far as relevance, though, only the couple involved can ultimately determine that. Marriage has been trivialized by more and more people being raised in the thinking of the Me-First-Fuck-You generation. Can't have a marriage when all you give a shit about is you.