Originally Posted by Carol H
As the caps are a decorative item and as I am a female, I like to replace the caps as they fail.
I'm not female (at least last time I checked) but it is a good idea to have a few caps on hand to promptly replace any that fail or come off.
The caps are sacrificial in that UV from sunlight will degrade plastic, the caps degrade but protect the cup. If the cap is left off the cup deteriorates, and the cup is helping to seal the hole the rivet goes through.
Water can get through the tube of a rivet without a cap, but the amount would be tend to be tiny unless the center shaft of the rivet was gone.