After spending some time on http://www.dictionary.com (all this time I’ve been using http://www.m-w.com. I’m a convert now), I had to respond to a couple of rants over word usages.
According to this, the popular definition of decimate that you imply is incorrect is, in fact, correct. The definition has broadened from the specific meaning ‘to reduce by 1/10th’ to ‘to reduce by a sizeable portion’. While the first definition is, as you say, the original definition, it is also the Latin definition. The second definition is the English definition.
This is a trickier one, because it seems to be based on the reader’s opinion. Dictionary.com presents a lengthy explanation of the problem with the use of this word, but they don’t indicate whether or not it’s okay to use. However, they do explain how it can be justified.
My beef with both of these beefs is this: language evolves. Meanings and usages change over time. You might as well cry, “Why don’t people use thee and thou and thy anymore?!” You might as well get irritated at the people who call refrigerators fridges instead of frigidaires or iceboxes. ‘Nice’ carries the meanings ‘fastidious; exact; precise’, but rarely do people use it in those ways any more. In time they’ll probably be listed in the dictionary under obsolete meanings.
Keeping a closed mind about the way you think the English language should work will often make you mad when somebody breaks the rules you follow, because the rules everyone else follows change. With language, it often is the popular opinion that governs rules, instead of what a select few believe.
[sub]I will admit there are some word usages which aren’t that fluid; words such as ‘ensure’ and ‘insure’ have two completely separate and established meanings, and to use one when you mean the other is a violation of rules. But you have to know when to slide and when to put your foot down.[/sub]
Seeing something occur again and again can wear down on a person. There’s a skit on the show Kids in the Hall where a man can not stop using the phrase ‘I ascertain that…’ in every single sentence. It’s certainly grammatically correct, but the man uses it so often that it grates on his fellow workers horribly. That may or may not be the case here, but it puts me in mind of that. I’ll admit ‘hopefully’ is one of those controversial points that people ain’t gonna let go of anytime soon. I just wonder when you old fogies will learn to get with the times. 
