We know, that Alliance is joing Horde revolutionists in attempt to Siegie and ewentually assault Org.
And why this is bad news for Horde? Because politically speaking its weaking the faction. Such a major player in WoW universe as Horde should not need or allow help of Alliance or any other faction. This just shows weakness of the Horde, its shows that they cannot deal with situation themself and must rely on help of their enemies to clean up mess within.
The alliance contingent that will arrive at gates of Org will use Horde supplies, they will see Horde tactics, logistics etc. up close.
Varian might want peace treaty with Horde after Garosh's demise but others might have diffrent point wiev.
He may be "king" of the Alliance but dont forget about Jaina, Garosh head might be not enoigh for her.
What you think?
'Sorry for syntax, english is not my native.