I think it'll always come down to personal preference, but for me, it has everything to do with size.

What I mean by that is simply this: I'm short, at 5'7", but stocky, weighing around 177lbs or so. My dad and grandfather (surprisingly, we all went into law enforcement) are built the same way. For me, wearing a kilt sans belt looks odd as I have a fairly short torso, comparitively. The belt serves well to break up the upper edge of the kilt, and brings some better symmetry. I think this can kind of be seen in my avatar pic.

When wearing a waistcoat, the problem is also alleviated and then no belt is really needed.