While I will not say it is commonly used, there is a fair proportion of the UK's population that wouldn't raise an eyebrow at the use of "ergo" in everyday speech. Sure, they'd probably think "pretentious git" or "over-educated public schoolboy", but they'd know what the word means.
While a dictionary does "describe" language, they also "define it". That's really the original intent of the OED: not as a spelling aid, but as a means of defining what words mean. Revisions come along because new words come into existence, or the meaning of words subtly change. Look in the OED for 1950-ish and you'll find that the noun "console" has different meanings to the 2005 edition.
(Okay, I'm going to put on the wonk-hat again, so be warned)
Slave has had various meanings during its long and illustrious career. Originally it was used to denote those who were settled around the German border by the Holy Roman empire circa 800AD to act as a buffer between the Huns and Rome. (Slav has the same root and is used to describe people in that area today).
Because these people were so often captured in the wars that raged backwards and forwards across that region, Slave came to mean someone who was held captive and (usually) held for ransom.
Meanwhile, a similar Old English word, meant "to work hard" ~ and, when these captured slaves came over here (probably with the French in 1066) it got superceded by the Old French esclave. In the UK we still use the original Old English form "slave" in that sense: I slaved over a computer all day. "Serf" is also derived from the same root word.
It wasn't until the 13th century that "slave" came to mean "a person who is the property of another" ~ a meaning it has kept up until this day. But, with the rise of computers, "slave" has taken on a new meaning ~ a subordinate device, controlled by another "master" device.
So, dictionaries not only describe words, they help define them too. Sure, it's definition after the fact, since it is the living language that changes...