Am I obligated to pay for two plumbers when one didn't do anything?

On recent plumbing work, one plumber showed up and looked at what I wanted done. He said that he couldn't do it by himself and would need to call his assistant. When the other guy arrived, he really didn't do anything but stand there and watch. However, I was billed for two plumbers work hours. I told them I wasn't going to pay for two because the other guy didn't really do anything. The guy that seemed to be the "boss" said that any workman on a job site has to be paid regardless of how much or little work thy do and if I didn't pay the bill in full, he would take me to court. Do I have to pay? If it comes to court, what are his chances of winning?