Being honorable is an essential point in Julius Caesar. In fact, the whole play is built on this concept. Therefore, it is essential that Julius Caesar, Brutus, and even people like Cassius are honorable leaders. This is what what makes the play into a classic and one of the great tragedies of literature, on par with the works of Sophocles. Let me delve into this more.
First, the conspirators plot against Caesar, because they question his intentions. What does he really want to do? Is he an honorable leader who will protect the Republic or a dishonorable one who will try to take it over? In the end, he is an honorable one, as his will is read and people realize that he was a good man.
Second, Cassius persuades Brutus to join the conspiracy. Brutus is put in a difficult place. If he joins, he goes against a friend. If he does not join, he may be harming the Republic, not something honorable. So, Brutus contemplates and joins the Republic out of honor. He is an honorable leader. He might have made the wrong decision, but his actions were based on honor.
In the end, everyone was honorable. This is why it is such a tragic play.
While the question of honor is integral to manipulating the actions of both senators and plebeians in Julius Caesar, the play's tragic conclusion hinges on
a) how the fetishization of honor can lead one to act against one's own interests and the state
b) how honorable conduct (e.g. being true to one's word) can compromise one's political career.
In Julius Caesar, Brutus repeatedly conflates honor with ego; he openly admits that he "loves the name of honor" more than he fears death. He does not love honor itself, but an honorable reputation. His efforts to preserve that honorable reputation while still serving his own ambitions lead him to shrug off his reservations about the murder of Caesar (he admits to himself it is necessary to "fashion" Caesar's behavior in public to suit his reasons for the murder) and to ultimately consign Rome to civil war, as his "honor" leads him to ignore Cassius's more pragmatic politics and spare Mark Antony, ensuring the collapse of the Senate.
Thus in Julius Caesar, while Brutus is firmly persuaded that being honorable is a necessary quality for a Roman, the occasional incompatibility of honorable behavior with effective political leadership is a theme in the play.