IF people stay until the end of the dungeon... they get some Valor!
IF they quit as soon as they have the item, you have a two second wait.. then you get another player who gets Valor!
You've never joined a dungeon on the last boss and thought "Sweet, I only have to do one boss for this random and I get my valor" ???
Well if you have you can thank people that quit half way. It's not like you EVER have to wait more than 2 seconds (unless it's a tank.. and then maybe 20) for a replacement, so I can't see how anybody could get upset about this unless they didn't get any loot and are pissed.
I drop out of groups all the time if I don't need the valor, with the nice thought that whoever takes over after me gets a short dungeon.