I suppose I would pose the opposite question to you -- is there a reason someone should only have one person who shoulders all their emotional needs, satisfies them intellectually and spiritually, as it were? I think it's easier on my husband, her husband, her and me for us to not be putting everything on one person. It would just never have occurred to me to put her to one side when I got married, or to her to do the same with me. We like having two very close friends each, one we have sex with and one we don't!
Well, I actually agree with you on a lot of this -- I get impatient with the frequent posters on here who sniff about how your best friend should be your husband, though it comes up particularly on posts about male-female friendships (one of my best friends is male), and I find the idea that you should 'step back' from any close friendships (opposite or same-sex) on marriage quite mad.
And I agree too that it's unfair in many ways to expect someone to be your sole emotional outlet, source of emotional/intellectual sustenance etc and my friendships are extremely important to me, and probably make my marriage more successful. But you're the one who has set up the either/or 'choice' between BF and husband, and said that if push came to shove, you'd choose her over your husband? (And I know the burning building was a bit tongue in cheek.)
My close friendships are consumingly important to me, but I would 'choose' none of them over DH. Two things -- one, is that I don't have a 'best friend' as such, I have a small number of individual close friendships as 'best friendships', which I prefer, and the other is that I think some of this must depend heavily on life circumstances.
I have been with DH since we were undergraduates, have moved around the world a lot, usually with him, since then, and we've made major life decisions together, with each other's happiness and preferences as a major component in this, had a child together etc.
This to me that he is the person with whom I make big life decisions, live with, and the parent of my child rather than sex is one of the big distinctions between my husband and my closest friendships. I will discuss these major life decisions with my best friends, and be sad that moving on may take me further away from them (as a recent move has), but these decisions don't fundamentally involve them.