In the case of a bilge pump, a check valve would normally be in the closed position, and could corrode in that position, which would be the worst case because it would render the bilge pump ineffective if a hull
leak also should occur.
The subject situaltion is not terribly dangerous because the through-hull is above the waterline except when wind would heel it over. Also with a proper loop installed the vertical the line should drain from the loop downward to the through-hull even without an anti siphon loop fitted. If one is fitted, any doubt should be removed.