Rhetorical: The essence of Benjamin’s argument is in the first and last clauses of these two verses (this single sentence!). Even were they to give all thanks and praise, they would be “unprofitable servants.” What is Benjamin’s reason for making the relationship appear impossible? Why tell them that after all they can do that they are “unprofitable”?
Once again we must remember that in the master/servant relationship there are actions and benefits that flow both ways through the relationship. A master may have a servant whose talents are such that the servant becomes indispensable to the master. In modern sports terminology, there may be a “franchise player” who is so valuable to the team that he must be treated with special rules. In this case the “franchise player” is a “profitable” servant because he contributes tremendously to the benefits that flow to the team owners. Benjamin is simply proclaiming that the benefits that flow from God to his people are so great that no servant may ever be “profitable” in that his contributions would begin to equal the benefits from the master. Benjamin is teaching the principle of grace, couched in terms of the servant/master relationship.
In Luke, the Savior uses these very terms to teach his apostles: “(Luke 17:5-10; 5 And the apostles said unto the Lord, Increase our faith. 6 And the Lord said, If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you. 7 But which of you, having a servant plowing or feeding cattle, will say unto him by and by, when he is come from the field, Go and sit down to meat? 8 And will not rather say unto him, Make ready wherewith I may sup, and gird thyself, and serve me, till I have eaten and drunken; and afterward thou shalt eat and drink? 9 Doth he thank that servant because he did the things that were commanded him? I trow not. 10 So likewise ye, when ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do.”
The Savior uses the very same servant/master relationship to help explain to the apostles what they need to do to increase their faith (the reason the parable was given, see Luke 17:5). While modern readers might wince at the nature of the master/servant relationship, Jesus simply accepts it as the way things work, and uses that common example to teach a point. The point is that the servant has obligations, and receives benefits for fulfilling those obligations. However, a servant who only fulfills the obligations is “unprofitable.”
In Jesus’ parable the point is to show the apostles a way to increase their faith, for Benjamin the same relationship is used to emphasize the grace of the eternal master. In both cases, it is the understanding of the relationship that allows the readers to access the meaning of the speaker.