This is a series of responses I had with a commenter or I thought worthy of a post of it's own:
C: Anyone who has eyes, whether he lived three thousand years ago, 200 years ago, or today, can look at a chimp and a human side by side and conclude that they are related. So what additional ammunition is there knowing that their DNA is so similar? It's icing, but not much else.
SA: Oh come on C, you've got to be kidding? Anyone who has eyes can look at a shark and a whale and conclude they are closely related, and he'd be dead wrong. You've illustrated the problem with casual observation as science. Its one of the fundamental problems with evolution-deniers, and with denialism in general - a lack of understanding of what constitutes evidence and an appreciation of what it means.
Scientifically, when you or I, based on casual observation, conclude that chimpanzees and humans are related, or that sharks and whales are, we've just gotten started We've done no science yet. All we've done is form a hypothesis. Without some falsifiable testing, it's nothing better than a guess. Indeed, that's as far as ID gets, and why as science goes its useless crap.
Science demands we one go further. From our hypothesis of close relationship, we'd need to make a falsifiable prediction, something that could give us a variety of answers, many of which would clearly indicate a flaw in our theory. (!) Let's check the DNA. If they are closely related (we'd reason), their DNA should be pretty similar.
So them we open them up, and lo and behold, the shark and the whale don't have DNA nearly as similar as we'd expect. In that case our hypothesis is disproven. But with the chimp and the human, the match is nearly total (95-99% depending on how you want to count it). This would be a confirmation of our theory. It doesn't prove it, but it gives it a huge leg up on any theory that hasn't passed such a test.
Empirical, falsifiable, repeatable testing of a theory, and adjustments to account for those results when they differ from prediction, is what makes science different from other epistemologies. The the rate at which mankind's knowledge grew once science became the standard is why it is held in so high a regard today, and why even those who criticize it with their words crave its stamp of approval with their actions. It doesn't get any better.
As long as you see THE thing that makes science the power it is as mere icing, you'll never understand science or the perspective of those whom practice and appreciate it.
Finally, it pays to remember that we knew nothing of DNA in Darwin's time. Some of the most enjoyable parts of "The Origin of the Species" were watching struggle to understand how traits were passed from one generation to the next. His theory demanded a mechanism, but he had none.
When DNA was discovered Darwin was vindicated in a huge way. There was simply no reason that such a thing had to exist outside his theory. Gods could easily have created without it. When it was ultimately mapped out so we could compare one creature to the next, the scientific battle over evolution was over for anyone with access to the information and the education to understand it. There was (and is) simply no way to explain, with any sensibility, why it looks like it does one creature to the next except through evolution via common descent.
Creationists take advantage of the ignorant and confuse them as to just how certainly Darwin's theory, with many modifications through subsequent discovery, won its scientific war. But win it it did, and those who recognize it will continue to grow as they have over these last 150 years until this debate is as quaint a memory as those about the existence of germs.