I think most of the legal/financial high and low points have been mentioned:
- significant tax incentives when one partner makes a lot more than the other (as in the case of a SAHM)/the so-called 'marriage penalty' when both partners make about the same. (Originally created as an incentive for women to leave the workforce at the end of WWII)
- death and divorce stuff... inheritance, social security benefits, distribution of assetts on divorce, alimony, etc
- automatic medical power of attorney, ability to make end of life decisions including burial, etc.
- potential loss of state benefits that are tied to income or marital status
But, there's also a lot to be said for the societal approval and support that married couples get. It's sort of intangible and I can't quite articulate the value of it, but it can be huge. Even when you have a kid together, there's a difference between refering to your boyfriend and your husband in casual conversation in the way you are valued as a family. If he needed to go to a doctor's appointment and wanted you there, you would get a lot more respect and cooperation from people if they knew you were going to be with your husband. He'd have an easier time leaving work to be with his wife. Your family might take it all much more seriously once you have that paper. And, if things get rough, others are more inclined to help you work it out with a spouse and might just tell you to get over your boyfriend. (Plus, if you are so inclined, this is about your only chance to get some cool wedding gifts

)
DP and I have been together for 12 years. Marriage only became an option for us this summer and may expire this fall. It will not be recognized federally, so most of the benefits listed above will not apply to us. But, while our families totally take us seriously as a couple/family, having this piece of paper now that it's available is important. In some ways, when it was not an option, it wasn't an issue to them. I think it would be an issue if we could do it and chose not to. We have over the years thought about doing a commitment ceremony, but as deeporgarten says, life just gets in the way... we bought a house, went on a once-in-a-lifetime Alaska cruise, tried to make a baby (not cheap for us!), adopted a baby, are raising said baby and saving for college, team sports, etc and so on.
But, with the November deadline upon us, we decided to keep our aniversary the same and get married this October. now I'm in full wedding-planning swing... We are planning a modest party to celebrate the people in our lives who are important to us, who have helped us stay strong for 12 years and who we are counting on to help support us in times to come.
Anyway - sorry to ramble... best of luck in any decision.
Cyndi