The presence of a middle class perpetuates the idea that the poor are to be blamed for their poverty. For the wealthy to decry that the avg poor person should pull themselves up by their bootstraps to a place of wealth and prestige is far-fetched, if not outright ridiculous notion and neither the wealthy or the poor really believe such is possible on a mass scale. Therefore the Middle class services an important social cohesive function. The wealthy need the presence of a comfortable "middle class" as as a realistic outcome to justify their ethics to the poor.
However the poor probably don't need the middle class. The middle class serves a as buffer between the wealthy and poor. If not for the middle class the poor would have more more power to wield in labor and distribution. If the poor could boot strap a business to market directly to the rich, then the redistribution of wealth would be more direct, rich to poor and you would have more upward and downward mobility leading to less social stability. Both the rich and the poor have essentially little to lose in taking risk in our modern economy because of it's social class stability. No matter what you do today, you will probably be as rich or poor tomorrow as you are today. In a sense recklessness is encouraged on both ends because there is little to lose.
In contrast the middle class are then required to be the responsible and fiscally conservative ones that maintain the social stability. They are the ones that are expected to plan and save in times of plenty and then spend that saving to maintain economic continuity through times of recession.