Well, it's actually many Wiccans who call themselves Witches. My understanding of the history of the word "witch" is that it originally comes from the Old English word for witch, "wicce". In Old English, the "cc" sound is pronounced "ch" making the word "wicce" to sound like "WE_chay". The spelling evolved into "witch". Some speculate that the word derived from the root “wic”, meaning “wise”. (The linguistic theories about the origins of this word are uncertain, but this seems most likely)
The Witches, then, were "the Wise Ones" of a community. They were the healers, herbalists, midwives, shamans, or perhaps simply one with "the sight". They were often the teachers of tradition, and the interpreter of patterns.
Many Wiccans who are trying to reconnect with their ancestral, pre-Christian spirituality are trying to reclaim this powerful word and strip it of centuries of persecution. They wish to honor the earth wisdom that was found in their earlier predecessors. They are proud of the word Witch. They honor and remember the many men and women who died because of this label. They wish to reclaim the power and wisdom that has been buried under centuries of misunderstanding.
Personally, I prefer the more emotionally-neutral word "Wiccan" unless I know for certain how my use of Witch will be understood. Stereotypes build walls between people, and the word “Witch” instantly erects a barrier of misunderstanding. It helps people understand my point of view more quickly if I avoid vocabulary that will cause misunderstanding and confusion. Among other Wiccans, I use the word Witch freely, but in the "mundane" world, I say Wiccan.
~Flame RavenHawk