Because if something is true today, it doesn't have to be true tomorrow. Likewise, there is no problem with saying something is true only insofar as it helps me win an argument. I learned this yesterday on my last day at Friendly Atheist, when atheists ran about insisting that atheists never say religious people are bad or do bad things as part of their arguments. I mean, really? That's like Limbaugh saying he's never criticized Obama. Which he wouldn't say, but so often those on the Left would. Prize for the best exposition of this phenomenon goes to Dennis Miller, who points out the obvious.