It's worse than you think, since the air is seldom flowing straight on to the wheel. Any slight cross wind makes the wheel "side slip" in aeronautical terms, and then a sharp edge is a liability, since it stalls, causing a lot of drag, and as the drag vector isn't in line with the wheel it generates a lot of side force, turning the wheel and sending you into the nearest ditch.
It's worse than you think, since the air is seldom flowing straight on to the wheel. Any slight cross wind makes the wheel "side slip" in aeronautical terms, and then a sharp edge is a liability, since it stalls, causing a lot of drag, and as the drag vector isn't in line with the wheel it generates a lot of side force, turning the wheel and sending you into the nearest ditch.