Maybe, but there is an astonishing number of drivers who don't regularly check their mirrors.
So is it just redundant and unnecessary or an ******* move?
Unnecessary in my opinion. Either the person is not paying attention or they have an attitude problem which flashing them likely causes them to speed up and block you. I'm pretty impatient, if they are 3-4 car lengths past the vehicle on the right and they aren't moving, I whip around them. Frankly I don't care if it pisses them off, I don't Sunday drive.