First, God and Satan are not equal powers duking it out in the sky. God created Satan and keeps him in existence. If at any time God willed that he no longer be in existence *poof* that would be the end of him, no fighting or battle necessary at all.
So why does God allow him to keep hanging around!!!
Because God is the ultimate teacher and He knows humanity. He really knows humanity, with a kind of knowing that we only dream about.
If you think about how children learn perhaps you will catch on to why Satan is necessary. Children learn opposites. If you teach a child the color pink, without teaching it any other color then it will awake to the realization that everything is pink and be quite pleased with its knowledge.
None of our words would mean anything unless they had an opposite. Think about anything, color, temperature, emotions, taste, noises. Try to describe any of it without using the opposite or difference of another thing and you have nothing left. That, in itself, puts Satan and evil in a new perspective.
I form the light, and create darkness: I make peace, and create evil: I the LORD do all these things. (Isaiah 45:7)
As for you, you meant evil against me, but God meant it for good in order to bring about this present result, to preserve many people alive. (Genesis 50:20)
Is it possible that just as Joseph's brothers plotted evil against him while God plotted the same thing for good, that Satan plots evil against us while God plots good? Is it possible that Satan is an unsuspecting tool that God uses to bring us to Himself?
"It doesn't matter, really, how great the pressure is; it only matters where the pressure lies. See that it never comes between you and the Lord--then the greater the pressure, the more it presses you to His breast." -Hudson Taylor
One last thought: The evil, or suffering that God divinely planned for our lives does not press only ourselves closer to God. I believe that as Joseph tells his brothers ". . .to preserve many people alive." So the suffering in our lives serves a dual purpose, to preserve many alive. As is prophecied in Revelations, "They overcame him because of the blood of the Lamb, and because of the word of their testimony. And they did not love their soul until death." (Revelation 12:11)