Most knights would have a strong sense of duty and honour.
The reason for the oath is to try and keep them out of politics, driven by issues early in the empire. The emperor can't use the knights to bring a clan into line, except by withdrawing their support.
That's pretty much what I meant - the oath keeps them out of politics. Any Knight who uses his military power to get into politics (hopefully) delegitimizes himself in the process.
Have there been any coup attempts by Knights? Or do they have a pretty spotless record?