Obviously, we should diplomatic relations with this important oil supplier. Not listening to them would only invite international disputes and lead to conflicts. It's important to have diplomatic relations with all countries, especially ones that are important for our energy security. Saudi Arabia is an important ally in the War on Terror.
Diplomacy is need in order to preserve trade security with any nation that the United States does business with. To argue otherwise would be to say that the United States can enter any nation and do whatever it wants without the expressed permission of some sort of governing official. One can certainly have issue with the type of diplomacy that has been used in the recent past or with the governing officials consulted, but one cannot argue that diplomacy is not neccesary.