IMO there is no reason NOT to ride it as long as the nut is torqued to spec. Almost every other bike on the road (jap or american) has nothing but a nut on top with out the clamping action. Not to mention that it is still clamped to the fork tubes on each side. I would definitely replace it, but I wouldn't hesitate to ride it as is in the mean time. Of course I'm assuming there are no other problems to go with this. Lay off the wheelies if it makes you feel better.