"Power concedes nothing without a demand—it never has, and it never will."
Lesser-evil voters fail to make any demands of the least worse candidate. It's as if they're saying to said candidate, "Here, take my vote. I ask nothing in return, except that you not be that other (more evil) candidate."
That's bad civics. It's bad citizenship, and bad politics. It's also bad history. Every single progressive right that we have today was championed first by forces outside the duopoly, and became so popular that it forced either of the major parties to adopt it outright.
Lesser-evil voting has brought us nothing but progressive degradations in our democracy, and an ever-tighter ratcheting of the corporate coil around our collective neck. No thank you.