Why do you think the slaves defend their masters’ goodness? Is there really any benefit to having the richest or smartest master?

The slaves defend their master's goodness because they can never be sure that the person they're talking to will not report their words...... fear of reprisal.

It is partly in consequence of such facts, that slaves, when inquired of as to their condition and the character of their masters, almost universally say they are contented, and that their masters are kind. The slaveholders have been known to send in spies among their slaves, to ascertain their views and feelings in regard to their condition. The frequency of this has had the effect to establish among the slaves the maxim, that a still tongue makes a wise head. They suppress the truth rather than take the consequences of telling it, and in so doing prove themselves a part of the human family. If they have any thing to say of their masters, it is generally in their masters' favor, especially when speaking to an untried man.