I for one have tried cg_drawgun 0 and I can't play like that. It hides all kind of useful feedback: flamer flames and mara zaps, timeouts caused by reload; plus I discovered I rely on the weapon model for aiming too, not just on the crosshair.
Regarding this last point, I can't treat a FPS like an exercise of clicking the mouse pointer at certain places on the desktop (ie. placing the crosshair on certain things). That's a pure 2D exercise, whereas a FPS is 3D and I tend to consider the weapon model an extension of the mouse hand. It's how I develop the particular muscle memory involved.
All the more reason why lefties would want a left-handed weapon model.