Hello,
the reason why vir bonus was so important to Cicero and Quintilian during the time when Rome changed from a republic to an empire is due to the fact that single men, or single governments, were now beginning to effect the path of the nation, and the people had less of a voice. The importance of good rhetoric no longer applied to the population, but to powerful men in the government.
Cicero was especially afraid of this, as he insinuates constantly, and rather negatively, toward Caesar. This is very clear during De Orate when cicero writes, “Ye there is no doubt that, even from our state alone, we could produce an endless list of absolutely outstanding leaders in war, but could name barely a few who have excelled in oratory.” He fears that few “good men,” as in ones who excel in civic virtue and oratory, will be involved in government and bring Rome back to a republic.
Quintilian, in his works, carries on a similar line of thinking. To him, a good man was one who excelled in oratory and spoke well, especially from an early age. Yet, also, that same man must not abuse his talents, and instead use those skills for ethical use only. He too felt the need for knowledgable orators, or “wise men,” in the government spheres. If the “wise men” did not use their talents ethically, then they could lead the nation astray through their oratory because of how powerful it was in government and nowhere else.