I think marriage, as with all ceremonies, is a good tradition to have, purely because of what it does. It draws two families together in a way that would not happen if it didn't happen. It lets you see odd relatives that you thought had died years ago, meet people's new partners, and catch up that perhaps you would not do normally. I think even in this age of Facebook, that sort of thing is still needed.
I also believe in marriage for keeping a couple together. Not in a "they wouldn't stay together unless they were married" or even that a couple needs a piece of paper to keep them together, but in a realistic sense if you have built a life together, and one of you dies, there's a heck of a lot of paperwork to be done if you're not married. If you're married, it just goes straight (!) through. Same with children - if parents of a child are living together, have been together years, or whatever, this means nothing to a court if the birth mother dies. Marriage makes things simpler, from coping after death to just getting a mortgage. Banks still see married couples as more stable, and you're more likely to get support if you can prove that you will stay with that person for a long time. Maybe the argument is that banks shouldn't do that, but I think there is more stability in married couples.