My Altmer supports the Storm Cloaks because she wants to sever the ties between Cyrodil and Skyrim so that the Thalmor can invade and use Skyrim as a staging area as Hammerfell was lost to the Thalmor.
By the eight! What have you done?! Drag the war as long as we can benefits the Dominion more. Failing that, an Empire victory is a better outcome.
On their victory, the empire still has to maintain a sizable force in Skyrim to prevent another Ulfric from rising. This keeps the focus of both the Imperials and remnants of the rebellion in Skyrim.
Should the empire lose, it would have retreated all available personal and give up on Skyrim altogether. Then we would have to content with one more very aggressive rival nation that may begin to focus their energy on retaliation on the motherland.
You have betrayed us! Beware the night, for the Thalmor does not forgive nor forget.