Yes, God has given us a promise of becoming judges, kings, and priests in the World Tomorrow, but He has not given us the authority to be judges now. We are to be making evaluations and learning how to judge. But, right now, unless we have been appointed to a certain position in which such judgments must be made, if we take it upon ourselves to make them, we are stepping into the muck. We have gone beyond our sphere.
He is telling us that, if we decide to take it into our own hands to judge another man's servant (think of it in terms of every other person being God's servant), then we have begun to be presumptuous. We are meddling in another's matters.
Jesus would have been meddling in somebody else's affairs had He decided to arbitrate the dispute in Luke 12:13-14. He would have been what the Bible calls a busybody—someone who is doing something that he has not been called to do or been given the authority to do.