I don't think it's disrespectful to lean your chair back.
I'm 6'1" – not the tallest and not the shortest guy on the plane. In most coach airline seats, if I sit straight upright (no slouching at all) my knees still hit the seat back in front of me, and that's if I spread my knees apart. And that's with the seat in front of me straight up.
If some one decides to lean his seat back, there's even less leg room, and their seat back is in my face. That's intolerable. People who decide they don't have enough seat room, and lean their seat back, rob space from the people behind them. That's beyond disrespectful – it's downright selfish.
It's nice to imagine someone asking if it was OK to lean their seat back, but I've never witnessed it. I've witnessed the opposite plenty.
Contributing to my mentality is that I'm a fairly small guy, so I don't get inconvenienced if the person in front of me leans back.
And that's the reason for the different opinions. Because I'm not a fairly small guy, I think the Knee Defender is a great idea.
They're not unlike those pocket sized cell phone jammers that used to be available. If you're trapped in a waiting room with a loud and thoughtless cell phone yakker, just reach in your pocket and hit the button. They weren't technically legal, but used with some restraint, they were effective at removing or silencing a nuisance.