Agrippa notes that some masters encourage that one should bend the arm when in what he calls Prima. He is very much against this for a number of reasons:

The straight line is longer and quicker

With the arm extended, your defense is with the stronger back half of the blade

With the arm bent, your defense must be made with the weaker half of the blade

Your arm and sword will be strong enough to withstand a blow, allowing you to defend and attack at the same time.

Nor does Agrippa like taking a wide stance while in Prima.

By taking a narrow stance, your body will be a 1/4 or 1/2 step further away

You can step forward or back without moving the left foot, running back, or sliding back

You can avoid an attack without having to parry

The images above are from Fabris and Capoferro, who published in 1606 and 1610 respectively. While the basic guards of Agrippa out-lived him, clearly the finer details did not.