When Squall pulls the trigger, instead of making an simple slash, the sword makes a sort of a deadly wound. Because the vibration makes the edge of the sword to open the enemy's body instead of just cutting it.
First, it's from a videogame, Final Fantasy VII, and about the pistol grip and that, you are right, it's uncomforable, but anyway the handgrip it's longer and more adjacent to the blade. For more information ask google xD or look here! [link]
Useful or not, Revolver Gunblade looks pretty cool