There are a number of things that bother me about this response:
1) It doesn't actually deal with the issue at hand. It's an evasion tactic that shifts the focus of debate from whether or not said behavior is actually acceptable, to "who started it". As such, it seems like basically a way of avoiding responsibility for one's own actions. So what if the other side does it? Does that thereby excuse your side from doing it too? Do two wrongs make a right? It's especially ironic when conservatives do this. I mean, aren't they supposed to be all about "personal responsibility", not just passing the buck?
2) The "your side does it too" response falsely assumes that I am actually on the "other side". This is not a safe assumption, whether theologically or politically. For instance, while I may be "post-evangelical", evangelicalism is still my heritage and the tribe I most easily identify with, so when I point the finger at the short-comings of evangelicals, it's not pointed at "them" so much as "us". And to be sure, I'm definitely not standing on the side of the mainline "liberals" either. Just because I've moved somewhat beyond evangelicalism doesn't mean I've therefore become a mainliner. That has become abundantly clear to me the more time I spend at my mainline Presbyterian seminary. I respect, love, and am intrigued by my mainline brothers and sisters here, but in many ways I still feel like an outsider looking in. So when folks tell me that "the mainliners do it too", my first thought is "Great, so what? I'm not a mainliner so what does that have to do with me?"
Likewise with politics, just because I no longer identify with the Republicans (though at one point in my life I was, literally, a card-carrying member) doesn't mean I therefore am a Democrat. I'm just as happy to criticize their antics as well (as I did, for instance, just the other day when I posted a Facebook complaint about how the Dems need to just let this Joe Wilson thing just drop.) Thus, when folks respond "The Dems do it too!" again my response is "Yeah? So what? They suck too. But shouldn't you be worried about your own side?"
Of course, I'm sure some might wonder why, if I don't consider myself either conservative or liberal in either of these spheres, why my critiques are usually directed primarily at the conservative side (which, I'll freely admit, they typically are). The answer is because, as I said above, the conservative side of things is where I come from and what I know. I can critique evangelicals because in some ways I still am one. And I can criticize Republicans because not too long ago I was one. However, not having ever been a mainliner, nor a devoted Democrat, I have a harder time pointing out their faults simply because I'm not as familiar with them. Indeed, it feels somewhat unfair and inappropriate for me to criticize them without fully understanding them. It's the whole "nobody better criticize my momma except me" thing. When you're part of (or have been part of) the family, you have a right to point out its faults. But when you're an outsider (and always have been), one has an obligation to understand before critiquing. (Which is one of the reasons I'm here at a mainline Seminary - in order to understand what makes mainliners tick so that I can better understand the critiques of my mainline Emergent friends.)
Anyhow, I'm sure I'll get flamed for this post, though of course it would be extremely ironic if the response to it was "well, liberals use this same evasion tactic too!" So let me be the first to say it, yes of course they do it too, and more than that, I'm sure I've done it too at times - I'm certainly not perfect. So why don't we all commit to dealing with the actual issues from now on, and not try to avoid seeing them in our own side (whichever side that is) by only ever pointing them out in the other. Let's all agree that "well, your side does it too", is never a valid counter-argument, especially when you haven't yet dealt with the fact that your own side is also doing it.