This seems unnecessarily discriminatory. It also means that taller people get a worse deal - already having to deal with cramped seating, now there is a premium on the extra body weight due to height.
As it stands, small and light people are being discriminated against: they are subsidising the large heavy people. People just don't talk about it that way, normally.
E.g. you might want to discriminate between smokers and non smokers in assigning health insurance premiums. This would be fair, but it would still be discrimination. Not discriminating would be unfair in that smokers are more likely to have health problems.
If nothing else I think they should charge more by the horizontal area you take up.
Ideally, we could computer model our desired seating arrangement and pay accordingly. If you don't want to pay for your waist-line, you don't travel.
But in general, airlines like tighter seat pitch as it lets them (perhaps) get another row into the cabin, and that's another 4-9 passengers per flight.
Seatguru has a sortable grid of seat layout. I won't be on airberlin anytime soon (29" seat pitch, no way!)
I wanted to try that last December, traveling with Delta from AMS to TPA. Turns out, it was $120 more for each leg of the trip. So no, I chose not to, even though I'm 6'1". Instead, I got a seat at one of the emergency exits.
Also, I only snagged the exit row seat for the return flight. I don't qualify for Diamond level with Delta, as I fly with several different airlines (non-Skyteam). With Delta, I do qualify for 50% discount on upgrades, I listed the fees that basic members would pay.
The nice thing about this method is that business travellers could get lower pricing because they generally travel without much luggage. Plus my wife and I are generally 10kg under the allowance when we travel, so it could work out economical for me if bought to Europe.
But when you are charged extra because your weight is above X, it is discriminatory. So airlines should not charge extra but should charge by kilo-distance.
Weight distribution matters, particularly when you're in a single prop 8 seater. If some dude rocks up on the tarmac expecting to fly, but weighs as much as four other passengers combined, you'll have a hard time distributing that mass. Also, with light aircraft, your incremental fuel cost per pound of fat is that much more noticeable.
For the record, I'm both a pilot and a, um, Rubenesque, fellow.