Well I see the point of friendlyness still.... Like I got sch book drop in T9 roll need for vanity. Someone asks friendly if he can have and as I know its just vanity I have the possibility to leave the instance to "pass" it for this person.
Also in instances like WoD and Co it often prevents people from leaving after looting.... This point is discussable I know but I personally run a dungeon to the end even when I got my loot (unless raid is calling and I get a free penaltyless jump because someone else left before me :S )