marriage = commitment?

I was reading the Parade this morning and ran across an article about Angelina Jolie and Brad Pitt, specifically how they aren't planning to get married. They want more kids together, but have both failed at marriage before and therefore don't plan to tie the knot. Parade disagreed with their choice.
So here's my question: Doesn't having kids together - and I don't just mean CREATING an child together but actually raising kids together - make a stronger bond than a wedding and the rings you wear? With the divorce rate somewhere around 50 percent, is marriage the symbol of commitment it once was? Or is it more a traditional value that we'd like to pass along to our kids? And I'll go ahead and cross that line - the gay and lesbian couples Chris and I know are no less committed to each other than we are, even though they can't get married.
For that matter, I know some male/female couples who raised children together before finally getting married. The reason they delayed getting hitched? The cost involved with a wedding, for one. Other couples I know were acutally brought together by the unexpected advent of a child and only got married after they were sure they weren't just "staying together for the kids." They have a strong marriage - now.
So what is marriage? And are couples who live together and raise children together any less committed to each other than those who plann a wedding together? Keep in mind that if Brangelina stay together for seven years, they're married in the eyes of the state anyway. And it's not like Angie's ever been a traditional girl.
--Misty