First of all, it's what people believe a word means now that is important, not what a Roman thought it meant 2000 years ago. If we're going to worry about Latin words having changed their meanings, what about all the English words that don't mean what they once did?
On the Chrons, sometimes people ask for a word that could be used in a setting in the past, and people provide various suggestions. It then turns out that what someone three centuries ago meant when they used one of those words is not what we would now understand by it, thus making what sounds to us like plausible speech and narrative of old to us no such thing.
Secondly -- and I think that this is being ignored here -- most of us do not write in third person omnisicient but in either first person or third person close. How many of out PoV characters would know what decimate originally meant, or even that its roots were in Latin and in Roman military discipline? If an author carelessly uses decimate to mean "almost annihilate", what are the chances that the PoV character would also do so... or would not even know that the original meaning of the word was not that? Are our attempts to "up-educate**" our PoV characters -- or, to put it another way, flagrantly ignore the reality of their lives -- any less problematic?
The beauty of English is its mutability***; if one wants to write in a language where the menaing of words is policed, French, with its Académie française, might be a better bet.
** - There's probably a more elegant verb available.
*** - This still has the meaning that the Romans*** would have understood. (I say "Romans", but there will be words in Latin that may not have been in use when the Roman Empire -- the western one that usually comes to people's minds, not the eastern one -- was still in existence.)
