Nope for me. SMGTe is plenty high enough to be seen, and like others have said, drivers seem to be far more cautious going round me than they were on my wedgie.
I've always thought of a flag as a long distance marker rather than anything else. I can't see how you'd implement a flag to ensure suitable overtaking space. Horizontally, maybe?
I briefly had a flag from the London freewheel, but it tended to cover my rear light so I took it off again. My rear light is clipped into the top of the seat cushion, so its actually a fair amount higher than that on most DFs. Result is I'm often spotted earlier by cars in the dark, and you can hear the revs drop as the car goes "what-the-hell-is-that-thing-its-too-tall-for-a-bike-but-too-short-for-a-horse-maybe-its-a-policeman."
I guess motorists are just like horses in that regard. If they see something they don't understand they assume its a predator.
I don't ride a lot of dipped / undulating single lanes, so I'm less worried about disappearing between humps, and there are enough horse riding schools around here to make flags a little irresponsible for the reasons above.