Yep, since those witches were introduced in the Pretty Scary Update, players have started finding uses for them. Since witches drop all sorts of useful items, including bottles, Glowstone dust, gunpowder, Redstone and sugar, some players are beginning to farm them just as they would sheep and pigs. Battery farm them, actually. Ughh. Click through for heartless barbarism.
If you prefer a straightforward instructional video, then this is a useful guide…
…that said, this is the one highlighted on the Minecraft forums where, worryingly, very few people are pointing out the inhumanity of it all.