Friends are friends, but your lover is your lover. And I think I'd like to keep it that way.
This is not to say that you shouldn't be friends with your mate. I mean, c'mon - if you can't be friends, how'd you end up as a couple in the first place?
But, I was friends with my ex-husband. Good friends - best friends. We laughed, had inside jokes, fun memories that made us smile and even agreed on how to raise our kids and where we wanted to be in 10, 20, 50 years. But, we were never really a great married couple. There were good times, don't get me wrong, good years - lots of good years. But they were "friend good" not "lover good."
This is the difference as I see it: Friends get along well, spend lots of time together, and agree on almost everything. They can work as a team towards thier shared goals. But lovers value the intimacy they share and protect it with everything they've got, they support each other towards reaching both shared and individual goals in addition to all the other stuff friends do.
These are just some of the reasons I feel you shouldn't "marry your best friend." You sohuld marry your lover.