And you can't even delete users through the MediaWiki software. I would have to manually go into the database and remove the individual user tables by hand. Really more trouble than it's worth. And I don't see what it's worth, either. So it's really just trouble.
Why delete people? Steve is right. More trouble than an amount of nothing, which is of course trouble, resulting in the awkward kind of "trouble squared" math that causes people to drop out of high school.