"Who among you would say to your servant who has just come in from plowing or tending sheep in the field, 'Come here immediately and take your place at table'? Would he not rather say to him, 'Prepare something for me to eat. Put on your apron and wait on me while I eat and drink. You may eat and drink when I am finished'? Is he grateful to that servant because he did what was commanded? So should it be with you. When you have done all you have been commanded, say, 'We are unprofitable servants; we have done what we were obliged to do.'" (17:7-10)
Servants are always servants in front of their masters. They may be different among their own friends and in their own household or when they are on leave. They are servants in their work place and they have their duties to complete and the master counts only completion of works. The master does not watch on anything else except him as a worker and the work completed.
This is in no way implies that a servant is a slave who does not have his own free time to do as he wishes. Servant has his own time of freedom in which he could eat and drink. Everyone has his own time to act as he wishes and the time of work in which to act as the master desires.
This parable only goes to tell the disciples that they should feel like the servant in this story. They are given works to do and they should only feel that they are servants and they are obliged to do their work only. In short the disciples should not feel that they are masters. There is only one master and all the rest are servants. The disciples are obliged as servants.
