For Plato, the spirited part of the soul is potentially the most dangerous. As an avowed rationalist, Plato believes that it should be suppressed by the calculating, rational element, which should always be in control. Men are emotional, spirited creatures, to be sure, but they should never be governed by their emotions. For the thinking man in particular—the man of reason—it's essential that the rational element of the soul should prevail.
Plato's relative denigration of the soul's spirited element is reflected in his political philosophy. In Republic, his most famous work on politics, Plato argues for the ideal state to reflect the tripartite division of the soul.
In Plato's projected system of government, the spirited element is to be represented by the city-state's armed forces. Soldiers, of necessity, will need to show spirit, drive, and passion if they're successfully going to defend the state from outside attack. However, the army itself must never be in charge of the government as a whole. That role is reserved for the men of reason: the philosopher-kings. Just as the rational element must govern the soul, so too should the rational element in society govern the state.
This is a good question. In Plato's view of the person, people have three parts. The mind or reasoning ability is highest part of a person. This is the place of philosophy. Then there is the spirit or the spirited part of the person. This can be viewed as the seat of emotions. Finally, there is the appetitive part of a person, which is motivated by desires.
Within this tripartite view of a person, Plato believes that base people are controlled by their appetitive parts. They simply do what they want to do without any thought of what is right or wrong. There is little reasoning ability. They even rush headlong into their desires, because the mind does not check their hearts or the spirited parts of a person.
Hence, according Plato, what is most important is that a person is controlled by the mind. A truly good man is ruled by reason or we can say philosophy. The mind, then, must control and reign in the appetitive part of a person.
Finally, as for the spirited part of a person, it is seen as neutral. It can be base, if it follows the appetitive part of a person, or it can be noble if it is guided by the mind.