Thanks all. I understand better now. The cover by definition, contrary to what I was thinking, does not necessarily preclude water intake, but rather by design mitigates the intake as I see in the picture (depending on the situation with the valve below it). And that's just how my cover looks, although I'm not sure I can visually see those vertical sides, but I do see the plate and screws.

It would be interesting to know the history, i.e., why mine does not function as part of an integrated whole. Maybe a leak as suggested. It's also worth noting that apparently whether or not that main drain is completely closed off or not doesn't seem to affect the efficiency of water circulation, or problems associated therewith. Of all the posts I've read, I've never read "yeah, my main drain is closed because the valve leaks so I don't get good circulation and tend to get algae."

So back to a chicken-and-egg thing, if the circulatory system works fine without use of the main drain, why did they install them in the first place? And do they still?