This comes under the heading of the truly picky. Despite the fact that most dictionaries have caved in, some of us still remember that when the Romans killed one out of every ten (decem) soldiers in a rebellious group as an example to the others, they decimated them. People sensitive to the roots of words are uncomfortably reminded of that ten percent figure when they see the word used instead to mean “annihilate,” “obliterate,” etc. You can usually get away with using “decimate” to mean “drastically reduce in numbers,” but you’re taking a bigger risk when you use it to mean “utterly wipe out.”