Summary: When Jesus tells us this parable, He is not talking about "work ethics", He is talking about the grace of God and the Kingdom of God.
GRACE BEYOND MEASURE
One of the things that shapes our views and values is how things work as far as standards go. One of the standards that we adhere to is what we call the "work ethic". A "work ethic" is anything that promotes quality, effectiveness, character, diligence, promptness, punctuality and what is considered as appropriate in all of these areas as far as employment is concerned. We also think that salaries should be reasonable because we do not think that it is right for people to be exploited or taken advantage of by making less than what is at the least reasonable. We think that with the passing of time and paying our dues by working hard that raises should be considered as the cost of living rises every so often. When the system works likes this, we call it a fair system. We know that there are exceptions to the rule and that there are some people who do not always receive what we call a "fair shake", when they have been passed over when it comes time for a promotion or a raise in pay.
When Jesus tells us this parable, He is not talking about "work ethics", He is talking about the grace of God and the Kingdom of God. No one can argue that God is not gracious and generous with His grace. It is grace because it is a gift and not something that we earn.
THE OPPORTUNITY TO WORK
People who were in need of work would often be in the marketplace waiting for an opportunity for employment. It was in the marketplace where each of these laborers were hired throughout the different times of the day in this parable. They all had one thing in common, they needed work because they had to work in order to live. The owner of the vineyard came and hired workers at five different intervals of the day. The first four intervals were three hour segments from 6:00 am, 9:00 am, 12:00 pm, 3:00 pm and the last segment was the final hour of the working hours of the day which was at 5:00 pm. Although watches did not exist back then, these were or would have been the segments of hours that the workers would have gone to the vineyard to work. All of these workers were given the opportunity to work.
Life is a journey that is filled with opportunities. There are some opportunities that might not come but once in a lifetime. In his commentary on the book of Matthew, William Barclay notes this proverb: "Three things come not back----the spoken word, the spent arrow, and the lost opportunity." (The Daily Study Bible: The Gospel Of Matthew. Volume 1. Revised Edition. Philadelphia: Westminster Press, 1975, p. 371). There is nothing worse than a missed opportunity. Consider Matthew 20:6-7 (quickview)  "Then at about the eleventh hour he went out and found more men standing around, and he said to them, `Why have you been standing here idle all day?’ `Because no one has hired us,’ they answered. He said to them, `You go into my vineyard too’ (NJB). God is always providing us with opportunities. Consider Second Peter 3:9: "The Lord is not slow about his promise as some count slowness, but is forbearing toward you, not wishing that any should perish, but that all should reach repentance" (RSV). Beyond our repentance and reception to the opportunity of salvation, God wants us to serve in His kingdom. God does not want us to be idle in receiving the gift of salvation and He also does not want us to be idle beyond our reception of this gift in serving Him in His kingdom.