He is willing to work with them if he believes he, and his people, will come out the better from it.
Which brings up the question.
Will the war end with Garrosh's removal?
(Assuming Vol'jin becomes Warchief)
I see a lot (understatement) of Alliance players, and some Horde, believing that once the Garrosh is removed from power the war is over.
No more fighting each other.
We just shake hands and thats it.
Now before you all say "We have to work together to fight the Legion".
This is true we will have to fight as one to defeat them.
However the factions don't know this. They don't know about this great shadow that is heading straight for us.
So lets say Wrathion pops up at the end of the Siege and starts telling these 2 world super powers that they need to join up.
They have no idea who this guys is and why they should believe him.
If he tells them who he is that'll probably make things worse.
Old hatreds don't go away easy. Sure you get some individuals who can let go of them. But with the record the factions have I can't see it happening.
Both factions will continue to hate each other after Garrosh is removed from power. That won't change, and until something real happens they won't team up.
Maybe some Legion servants summon the first wave in secret and the Factions get attacked without warning, which causes them to have to team up (hopefully still showing the hatred between them though)
Just my thoughts anyway.