In the short term? Nothing. Nothing about this benefits the Outsiders. Mab is immediately stronger. Mab's forces man the gates. Mab wins.
So maybe we need to think longer term.
Mab might be the big winner from Aurora's plan, but destroying the balance would have wide ranging effects. As pointed out in Harry Johnstons answer, the relationship between the Fae and everything else is more complicated than the first books portray.
So, since speculation is required to get somewhere here, past the immediate short term, what might've happened? Summer and Winter would have kept on fighting on the battlefield of the Stone Table. Despite Winter now being stronger, the outcome isn't certain. First lets presume balance wins eventually... both Courts would have had to fight for a long time and kill off enough of each other to nullify Mab's new advantage and for balance to resume, making both courts vastly weaker. Weak enough to weaken the defenses at the Gates, or weak enough for Lea to take over (remember, Aurora is not the only infected agent, as KutuluMike points out). Either way, Outsiders win.
Let's take the other side of the guessing game - balance destroyed, Summer gone, Mab wins and rules all Fae. This doesn't seem good for the Outsiders, but consider what happens to Earth, the real(ish) world, in this scenario. Without Summer to counter Winter, a new ice age is here. Human civilization would probably go bye-bye. Humanity in general surviving would be a bit of a question mark. What we then need to consider is - what would that then do to the spirit world and the Fae? They're connected, after all. The Nevernever is half-formed from human perception. Gods have been destroyed simply by ensuring that they were forgotten. What happens when no one is left to remember the Fae? Even best case, Mab would eventually be greatly weakened. Middle case, what's left of humanity would rebuilt and create new legends, who end up manning the Outer Gates. Either of these would create an opening the Outsiders could exploit... and that's not considering the worst case, where nothing is left, and they can just walk in.
Bottom line it, anything that causes enough of a destabilization in the status quo could possibly benefit the Outsiders. As Cold Days shows, they are pushing hard now, and any weakness they can create, they will try to exploit. They've been doing this for a long time, and some of them are crafty enough to wait for a long string of dominos to fall.