As far as it's being rude, that's begging the question. I'm asking why it seems rude, because it just seems like an emotional response with no real reason behind it to me. I fully understand people think it is rude, but I don't really understand why it is worth being offended.
The next two just seem like hyperbole. How often do you really wait "ten to twenty minutes" in a dungeon without being able to continue? How often do you have to "adapt to a new player's style"? I have queued hundreds, maybe thousands, of times since LFD was implemented and I don't think I've ever had a group held hostage that long and I don't think I've ever had a group where we had to actual build some kind of cohesive actual teamwork in a five man that would be disrupted by having to replace one player. Most runs, people don't even talk to eat other and those spots could be filled with random players or bots and could cycle in and out at every pack and it wouldn't make a bit of functional difference.
I don't really get the third point. People get "rewarded" for exactly what they participate in. You beat the final boss? You get loot from the final boss. You beat four bosses? You get loot from four bosses. You beat only the first boss, then leave? You only get loot from the first boss, and not completionary gold/points.
And I just disagree with the final point. The anonymity is what causes this, not the ability to leave. You'll never see those people again and they don't know you. And it's arguable whether rolling on offspec loot is "ninja"ing anyway.
We can agree to disagree on the issue, but I really think that the people who are offended by people leaving halfway through the dungeon need to think long and hard about how much this "problem" really affects them and if it is really worth implementing punitive measures.
If you queue for a specific dungeon because you're trying for a particular piece of loot, this doesn't happen. You only get new instances. The only people that will come in are people who are queuing randomly, which means they probably need loot from a lot of places or, more likely the older the expansion, are queueing just for the points, in which case they benefit from a shorter run.
I said in my very first reply that people leaving mid-fight is a different issue. And that is something that can be countered by forcing people to wait until the fight ends, rather than taking away their loot or offering bribes.