And to be brutally frank, I can think--right off the top of my head--plenty of counterexamples. There are lots of people who are fighting in a ceaseless struggle into which they enter knowing that they're never going to completely and finally win, and in which they know they're going to sometimes lose some battles. Our government is constantly fighting a war on crime. In a sense, we're never going to win a "war on speeding" or the "war on child abuse." Those thing are never going to be completely eliminated until Jesus returns. Does that mean that those fighting it might as well give up?
Per usual, MacArthur puts it so much better than I can:
Church history is littered with examples of sects and factions who taught various versions of Christian perfectionism. Nearly all these groups have either made utter shipwreck of the faith or been forced to modify their perfectionism to accommodate human imperfection. Every perfectionist inevitably comes face-to-face with clear and abundant empirical evidence that the residue of sin remains in the flesh and troubles even the most spiritual Christians throughout their earthly lives. In order to hang onto perfectionist doctrine, they must redefine sin or diminish the standard of holiness. Too often they do this at the expense of their own consciences.