I regularly drive 50 mph in the slow lane (and *never ever, under any circumstances in any lane except the slow lane) when traffic permits. (ie, when the freeways are full of people driving 60, I will not 'slow down' traffic so I can find my speed - but if there are large gaps between cars and plenty of space to change lanes, I will drive my speed. At the hours I drive, that is most of the time).

This is dangerous

Not really. The freeway speed posted is between 55-65 mph. At any moment, you may be expected by law to reduce speed to 35-45 mph (something most people unfortunately do not do) due to the presence of a stopped vehicle or LE/Emergency Services. There's nothing unsafe about travelling at the speed limit, coming up on a car driving 50 mph, and then passing it. If you can't handle that then you aren't competent to drive a car.

and pointless.

My fuel economy increases from about 38-40 mpg to about 45-55 mpg (depending on initial battery charge state) when going from 60-65 to 50-55 mph. This is a major cost savings (amortized over time). Further, it is one I am expecting to realize having invested in an efficient car. Much as a person who drives a muscle car wants to accelerate in a way that some might consider unsafe, or some drivers want to drive vehicles that block sight lines in a way that some might consider unsafe - I want to drive a car that gives me fuel economy in a way that you apparently consider unsafe.

Why do some of you pull out in front of someone and not speed up?

I never change into a faster lane at a slow speed. This is unsafe and I'm not sure if it's against the law but I was definitely taught it is unsafe and against the 'rules of the road.' You always match speed with the lane you are merging into. I will say that it's much more common for a car to 'rush up behind me' that it is for me to 'rush up on another car' (implying that it's far more common for people to shift into a slower lane at a high rate of speed than it is for people to shift into a faster lane at a slow rate of speed).

Either way, if you are employing signals as you should this shouldn't be an issue - hazard avoidance is the responsibility of every driver, and shouting obscenities at other drives isn't a recipe for happiness.