You want rocks and dirt thrown up by the wheels to get into the torque converter seal area? You want chipped teeth on the flexplate?
Of course the cover is needed. GM would not spend the money to install it if it wasn't. The struts from engine mounts to converter cover make the engine/trans assembly more rigid.
I went out-of-my-way to reinstall the missing strut rod on my K1500. Had to grab one from a salvage yard, and dig the broken bolt out of the aluminum cover. Broken bolts on those covers seem common, I have two more covers that had broken bolts in them. Fortunately, it's not to tough to get the broken piece to unscrew leaving decent threads in the cover.