Yes, I find it disconcerting that next to every deck pipe (or I should say hawse pipe) that is sold with a cap-n'-chain there is a stack of replacement cap-n'-chains. That's why I'd prefer the hinged, spring-loaded cap. Have yet to see replacement caps sold for these.
The pictures seem to indicate that slits are made to accomodate chain, and that the lid would be propped open slightly if nylon rode were running out of it. Like you say, though, whatever water
makes it through will drain to the bilge