The whole 'drop the dialogue, defend yourself and start again' argument falls when the fact that the boss ahead is killed while we're nearby advances the quest, whether we're involved or not. I've had several instances where I'm talking to an NPC knowing there's a fight ahead. Someone comes, cue the red screen outline while in dialogue, and in five seconds I'm kicked out of the dialogue and the quest advances.
Wait. What were you saying about being careful about the whole situation? Nevermind that, the NPC has ran off already and I'm left thinking WTF was that all about.
Now I know better. If I'm doing a quest somewhere and there's someone else around, I stop. Wait for them to run ahead. If I see them fighting a boss I get the hell out of there, go back as far as I can to not be involved in the fight in any possible way. Make sure they are well gone and done, and only then do I advance and force the quest boss to respawn. It's an MMO. We shouldn't have to play defensively and act selfishly in order to enjoy the quest dialogue.
You're gonna get served the "It's an MMO" excuse anyway, because it's the truth.