There's usually a quest in instanced dungeons and I've found myself in a group, on more than one occasion, where the other players rushed the quest. Normally I wouldn't care (to each his own, right?) but some of the quests progress for the entire group and not just the player. This means that if someone in your party skips through the npc's monologue, it's also interrupted and applies for you.

I myself like to savor the game and go the extra mile to get the most out of it. I also believe the amount of work that went into voicing every every npc engages and encourages players to listen to what they've got to say. If someone can't be bothered and just wants to finish the dungeon without paying mind to what's going on, it's fine so long as it doesn't impact me as well. In this case I'd have to abandon and redo the quest in another instance to hear/read the quest giver's speech.

I understand there are different kinds of players, but I shouldn't have to miss out on content because someone else has a short attention span or finds it boring/doesn't care.