@S121
Because Vergil's Summoned Swords are so useful, and powerful.
Summoned Swords: They are not a Devil Arm!

Vergil's Summoned Swords are his equivalent for guns meaning they have no ammo. However, unlike guns, they do not recoil and therefore cannot cause him to hover in mid-air. When using Spiral Swords, the Devil Trigger gauge is reduced by two full runes. Furthermore, because summoning the swords does not interrupt the current action, they can be used at any time, even during recovery animations and combos. Not only that but they have a very decent fire rate, and decent power as well.
The moves you have with the Summoned Swords are also cool:
Spiral Swords: Create a barrier of countless swords which surround and protect you by holding down the fire button. It does high damage at a fast rate. The swords will vanish from around Vergil after they hit an enemy enough times.
Sword Storm: When using Spiral swords fire out the swords at the enemy. The swords surround an enemy for a few seconds then fire all at once.
Blistering Swords: Fire out swords in rapid succession at an enemy after executing a Spiral Swords maneuver. Basically turning the Spiral Swords technique into a machine gun of Summoned Swords.