I'm going to take a slightly different angle on the classic "free will defense" bit.
Perhaps God did give us free will, for the reason pointed out by Perianwyr and others: that love is meaningless if it is not given free. Of course, this also opened up the possibility for humans to do evil, and that possibility certainly came to pass.
The end reslt is that we do have free will. And to take it back now -which God could certainly do, if God is omnipotent- is not an option. To rob humanity of mind, the source of all human dignity, would be an evil which makes the sum total of all the atrocities humanity has ever committed look positively benevolent by comparison. Simply going back to retcon the decision has the same basic effect.
So the only way for God to eradicate all evil (at least, in the quick-fix manner which most people who make this argument seem to want to happen), now that the proverbial genie is out of the bottle, would be to do something far, far more evil than anything which has ever been done: the mass dehumanization of some six billion people, plus all their descendants. If God is truly omnibenevolent, then this is not acceptable.
But there is another reason free will is a necessary thing. After all, why would a benevolent God give us free will in the first place, if it would make evil possible?
At the risk of turning this GTKY, I will recount an experience of my own as an example. I am a Christian, and a rather devout one. For many years, I was the only Christian in my chosen circle of friends, and even now there's only one other. This, as I'm sure you can imagine, has caused a lot of friction, and even pain, for everyone involved. I see my friends doing things which I find to be immoral, and there isn't a damn thing I can do about it. To explain my views would harm my relationship with them. But to get too close to what they do puts my relationship with the God I worship at serious risk, for while associating with them is not bad in and of itself, it does draw me closer to committing immoral acts. I have undergone multiple crisis of faith, as I constantly reexamine my beliefs and my friendships, to determine whether I am still resolve to walk the path I have chosen for myself.
But I choice that path, just as I chose my friends, out of love and trust. And even though that combination brings a great deal of unnecessary complication and pain to my life, I choose to stay with both. That's what love is. And without free will -the ability to choose things even if they have immediately negative effects in hope of some longer-term gain- I could not do that, and while my life would certainly be easier it would probably be much less happy overall. Even if it would relieve the occasional short-term angst.
So, why do we have free will? Because it is what allows us to experience love. And yes, it also allows us to experience evil, but is that not a worthwhile tradeoff?
As for why I consider this different from the "classic" free-will defense: the classic argument simply states that God wants humans to have free will, with little explanation as to why it's necessary. I differ from this, by pointing out why free will cannot be taken out anymore: by doing so, God would become worse than anything He could hope to prevent.