That argument is a favorite of establishment Republicans who are in most cases in favor of amnesty, but who won't come out and admit it. So instead, they try to make a case that nothing can be done about illegal immigration and that it's a lost cause. You'll notice that they're never so defeatist on issues that actually matter to them.
I have seen it over and over again. It does not work with me, but every time the political class uses the “can’t deport x million illegal immigrants” argument, they get away with it.