It was my understanding that the reason for wearing the dirk was in case you wanted to go armed with something other than and in addition to the sgian dubh. That makes more sense than wearing one simply for looks and with what jacket.
I'm also told that there is usually a loop in the kilt to steady the dirk and keep it from flopping all over as they definately will if only suspended by the beltloop but I haven't found one yet. :smile:
There are differences in types of dirks also, there are plain ones and regimental dirks and I would imagine many types in between.