A guildmate of mine was healing a Culling of Stratholme run last night and finally broached a question that seems to have occurred to everyone who's helped Arthas take his utilitarian moral perspective on the road: "Why are we helping this guy?"
It's a question that people used to ask about Black Morass a lot too (indeed, the first boss, Chrono Lord Deja, will ask you that himself), but Black Morass was a little more cut-and-dried. Medivh unquestionably cost many lives in bringing the first Horde through his portal, but if the orcs never set foot in Azeroth, then the world would have fallen to the Legion. The Bronze Dragonflight is unusually blunt about the cause-and-effect; war breaks out among the human kingdoms, the Alliance never occurs, the new Horde is not present at Hyjal to defend against Archimonde's forces -- indeed, the Legion may very well have swept the world without Hyjal ever occurring. So, despite the destruction wrought by the first Horde's entry into Azeroth (and you could argue, because of it), Medivh must succeed in opening the portal.
I'm not sure it's quite that straightforward with "Old Strat" -- and questions about whether it is prompt some thought-provoking questions concerning Azeroth's past, present, and future.