This Sunday Gospel reading consists of two distinct parts; first, there is a request answered by criticism and second, Jesus offers a parable about a servant. Though our reading is verses 5-10, I think it useful, if not necessary, to intepret them as comments on the four verses which precede them. In these four verses Jesus teaches two difficult points. First, one must not scandalize a child; indeed, it is better to be drowned by a stone around one's neck than to scandalize. Such is the evil of scadalizing youth. Second, one must be ready to forgive some one who hurts, once that person has asked for forgiveness - and yes, there is no limit to the times one forgives in this situation.
The disciples realize how difficult is Jesus' teachings, and so they ask for a greater faith. Jesus' response means, not to encourage them or console them, but to criticize them, and thereby to let them know just how they stand before his two teachings. Indeed, if they had any faith, they could work wonders; clearly, they work no wonders, which shows how little faith they really have. Do they understand this about themselves? Do they understand what faith it takes to obey Jesus' teachings?
Jesus offers another consideration. A servant by definition has one role to play: it is to do what he has been asked or commanded to do. This is stern advice from Jesus, but again he is making clear to the disciples what reality is for them: to obey.
Can vv. 5-10 be interpreted on their own, without reference to 17, 1-4? It seems so, but their full significance for Luke is tied to these verses which precede verses 5-10. We know it is good to respect the logic Luke expresses as he puts one story next to another.
(In a number of translations we read that 'we servants' are to say we are 'unprofitable' or 'useless'. This cannot be the correct translation, for the servant in the parable works for his master all day; this servant cannot be called 'useless' or 'unprofitable'. Better is it to say that the servant should say that he has no claim on the master, that he has no right to expect anything but what a servant should receive.)