I'm an ardent traditionalist generally in life, but I never have quite understood the habit or custom of flying a host country flag, regardless of which halyard or spreader. If I drive my car to Illinois, do I add a courtesy Illinois license plate to my bumper? Say I'm in French waters, just as a random example. I'm certainly not a French boat, so why would I fly a French flag anywhere? Feels kind of shady or dishonest to me, like a deceptive naval battle maneuver in a Patrick O'Brian novel. I know, it's custom, and apparently very important or even required in some foreign ports, and some manuals or codes say it's a "duty" or sign of respect, but I don't get it. I even bought a Canadian flag for when we sail out of Duluth eventually and may turn north, but I'm certainly not Canadian and neither is my boat. And the average schmoe is not going to understand what that flag means on the starboard spreader or halyard.
The host flag doesn't seem to have a practical purpose of verifying that you've processed in for example and have permission to be there either -- you just take down the Q flag for that. And I recall I've read stories from after 1935 when foreign boats in German waters had to fly the Swastika, sometimes to great offense (thanks Bill Bailey, take that Bremen, but that's another story). But to address the original post, I agree with the others, If I'm in Dutch waters on a Dutch boat, makes no sense to me to fly a U.S. flag or ensign anywhere, even for sentimental or patriotic reasons. Not a U.S boat, or a U.S. owner, and you're not in U.S. waters.
I do have a flag of what is now Inner Mongolia though, because that's where Xanadu was located, and I'll fly it on the line from the bow to the forward bimini pipe when we dress the boat for the 4th of July, but nobody will know what the heck that flag means except us.
