Even the description of the test tag says not to use it, yet there are 40+ questions with this tag. Many look like they are from newbie users that aren't reading tag descriptions anyway, though it looks like at least one more experienced user has used it as well. Can't we just kill it?
To prevent this tag from showing up again, I've blocked the test tag:
The honest answer is we don't.
But removing a tag is not so easily done. Each question has to have it removed and still be properly tagged (even if it's closed). Simply put a tag that has questions on it can't be removed.
Once it's removed from all questions, then we can go about blacklisting it. And to blacklist it, we have to get Stack Exchange staff involved. So we really only want to do this once the legwork is done. They won't blacklist it if it is in use, so we really need our ducks crossed and our eyes in a row before we contact them. (Although they have keen eyes and will have probably seen this post before it happens, even if they don't comment.)