I find it extremely rude and arrogant.
It is to let the driver in front know that faster traffic is approaching
I'm scanning front, instruments, and mirrors. I don't need some wanker flashing his lights at me ... I already see him.
I spend 2.5 hours every workday traveling in moderately heavy traffic, on 4-lane divided highways, at speeds well in excess of the posted limits. I see a lot of cars, and a lot of drivers. I'd say about 99% of these drivers know how to get along ... the real hazards to the rest of us are not the left-lane slowpokes. Such creatures are almost nonexistent.
The real hazards are the headlight-flashing tailgaters and the lane-weavers.
These foul folk should be taken out and shot.
I'll flash to pass, but not in traffic, and not until I've given the squatter a bit to look up from staring at his hood ornament and check his mirrors...
Heavy or even medium traffic has different rules, but I get aggravated with people that sit in the left lane going 76 thinking "that's the speed limit, and I have no obligation to move."
Active drivers are not the problem; the problem is people that are doing everything else but driving.
Then you'll move over, I'm sure, and won't have to worry about it.