Another thing to keep in mind- valve covers for these things are a dime a dozen. 20, 30 bucks maybe. Plus, being aluminum, a crusty set for cheap will polish out nicely most of the time.
Heck, anything above the head gasket (valve train, heads, etc.) is pretty cheap and easy to get. And the design of the engine means the pistons never push out farther than the edges of the frame. It's a calculated risk, to be sure. But to me, if you get into an accident that requires replacing more than the top ends, crash bars wouldn't help anyway.
Again, all personal preference.