I would argue that all evil is necessary.
Being so does not make it any more excusable or any less despicable, but understanding why it is fundamental can make us wiser and stronger - and ultimately, therein lies the necessity.
If you knew no darkness, your appreciation for light would be subconscious. But once you have been in the shadows, you become aware. The light becomes a gift. If you knew no harm, security would be assumed. But after facing danger, you would understand the beauty of that safety. We can know good without evil, but we cannot fully experience and embrace it until we see the other side.
We forget that good and evil magnify each other: that in an easy life, the greatest evil may be your sister borrowing your clothes without asking, and in a harder one, the greatest good may be a warm dinner. We are forever at risk in this precarious balance between the two - risk that we should fail to see that balance and fall prey to the belief that it is unfair. Is it unfair? Should there really be no evil? Would we really rather have painless ignorance than painful wisdom? I say only if that wisdom goes unused is it more worthwhile to stay unscathed.
I spent several weeks in a psychiatric ward once for depression. I'd fallen prey to believing that there was too much evil in existence to ever know anything other than pain. Therapy did teach me a great deal, but I learned the most from the other patients. There was a boy there, younger than I was, maybe fourteen. He was an idealist, to the point of refusing to accept reality. I gathered that he was still in the hospital because of this incredible defiance. "It shouldn't be like that!" he would say, every day, every meeting, no matter how many times anyone tried to tell him that it simply was. True - we should not allow evil just because we recognize that we can learn from its existence or because we suspect that we can never eradicate it completely. But to so firmly refuse to see any greater purpose in it hurts us more than the evil itself could have.
Strength is overcoming pain. Wisdom is consciousness of both beauty and balance. In times of doubt, religious and spiritual people often wonder why there is sin and evil - why an all-powerful deity allows such terrible imperfection. But the truth is that the perfection is deeper and darker than we'd imagined. It is not in purity that we find it, but in balance. In strength, and in wisdom.