Marriage still has it's place as a sign of commitment. People normally marry when they are sure they found a person they can spend the rest of their lives with. These days marriage is taken far too lightly, and people get married for the wrong reasons, or they do it as a "spur of the moment" thing, just look at the high divorce rate. Divorce has it's place, but the rate is high for the wrong reasons.
It's basically a way for a couple to tell each other that they are so sure about this, that they are willing to take a big step. It's not the same as just "being together". If you are just together, she/he *might* leave the relationship with the first BIG fight. There is always a quick exit. If you are married, you work things out, because if someone leaves you break that symbolic bond. Marriage is by no means perfect, it requires work, but the benefits are beyond what I can describe.
As someone said previously, I think it is important for kids to have parental stability, although single parents do fine as well (I was a single parent child, but not due to divorce).
I would not give up my wife, marriage or child for anything in the world.
Edit: This goes for same sex marriage as well. I'm friends with a couple who has been together for 20 years (and they have kids).