Everyone is different, but I just think it's a little curious to categorically think that no one can ever possibly have more than one best friend. I would say I have four, and it's five if you also count my sister, which I do. But they are all people I talk to or text with on a daily basis, barring unusual circumstances; they are all the people I want to tell immediately when something great or terrible happens, they are all people who I know would take my call at 3 a.m. in an emergency, and whose house I would drive to at the drop of a hat if they needed me. We've travelled together, and mourned for dead friends and family together. Most of us have had big fights with each other that we figured out how to resolve in a healthy and mature way.
I've known all of them for over a decade, some for thirty or forty years. We have different histories, and different memories together, but I can't think of a single one of them I'd kick off the lifeboat if it came down to it. There are also another three or four who I have just as much history with, but due to geographical distance and personal circumstances we don't talk as often, but in emergencies, we've still been there for each other, and we still have in-jokes from twenty years ago. If I would say that if any of them have any failings at all as friends, it's that none of them are remotely interested in the sims. :D
I don't know how I would elevate one of those people over another. I mean, there are other people who I do consider casual friends or warm acquaintances. And I am absolutely not an extrovert, I'm definitely not one of those people who has several hundred Facebook friends, or makes new friends all the time. But the friendships that I do have are pretty deep and longstanding, so... I don't know, it would seem very arbitrary to me to elevate one of them over the others.
All of which is a very long-winded way of saying, it makes perfect sense to me that TS4 should have the option for sims to be BFFs with their spouse, and their closest sibling, and their college roommate, and their lifelong best friend from sims scouts, if you want them to.