In reading this verse I am reminded that I need to watch what I say about people at work. In an environment where decisions don't make sense and very anti-family and just anti-common sense, I tend to refer to those making the decisions as stupid or idiotic. But here Jesus tells us not to call others Raca. Which most all teachers of scripture agree means empty-headed or useless person. How being subject to the courts applies to use is unknowable since we have no such laws against speaking these words. We must assume that if Jesus spoke of us being liable to , back then, earthly courts then surly we are liable to a Heavenly court. But weather liable or not, just because the Lord said not to do it is enough to warrant compliance.
In context this passage is in the "Sermon on the Mount" which deals a lot with judging others and our attitude to those over us. We seem to be told to put our trust in those over us as a sign of our right relationship with the Lord. I suppose if those over us make a mistake and it affects us negatively, then we must in turn trust the Lord to work it out to our or His benefit. Again the Kingdom of God is not about me but Him who rules it. Trust, what a wonderful attitude.

Matthew 5:22