As a flasher, I understand when there are situations which require a much slower driver to occupy the left lane for overtaking another vehicle. I will not flash in this situation and I typically give the overtaking car a good 5-8 car lengths distance to make the pass. When you overtake the slower vehicle, you will typically move right. Then I will overtake you.
Tailgating, as an overtaking car, is just as dangerous as the person who sits in the left lane bottlenecking traffic.
I don't flash the second I get up behind someone: "HEY, GET OUT OF MY WAY! I'M IMPORTANT!". It's more like after it's clear that they're simply driving side-by-side with the car next to them and there's no reason to block the lane: "Excuse me, I think you haven't noticed that there are people behind you trying to pass."