Let me put it in a different way.(partly rehashing #43)
If at any time you enter a physical overhead you need to run a line. If your experience tells you that you will be okay to exit in the worst case scenario, some poeple opt not to put one in. I am NOT saying don't put one in, I am saying know what you are able/capable of and how you will handle the worst case scenario. If at any doubt, you need to put one in
or dont dive there.
What is that worst case scenario: (and I call them out separately, but they could be joined)
-silt out, loss of vis
-black out, loss of lights. Including the sun/daylight
-loss of direction, just plainly lost
-your buddy (who knows the system) is lost/separated, now you alone must make it out.
Bottom line: you can't see the exit nor do you know where it is. How do you get out?
(and I haven't even begun to discuss the other compounding factors)
A line in an overhead environment is not to get you back, it is to get you out. This can mean returning to the point of entry, or a traverse. How ever either of these two looks on a map, you need to get out.
In the recent posts people are talking about line retrieval constructions, debating if/how.
If you are debating that, you may not be ready for an overhead swim. My personal view is to drop a line in and leave it. A few feet of line off my spool/reel is worth me getting out alive. But make sure it is put in neatly, you don't want it to snag some other diver.
A tubular, large sized swimthrough for me personally does not request a line. Any deck on a wreck does, it does not matter how many times I have been there. In a spree of Murphy's tea party, any well seasoned diver (anybody in the team) may make a mistake, a solid line may be the only lifeline out.