For a few years the new username lists were trawled by vandal patrols and there was a lot of biting of newbies -- so much that "don't bite the newbies" had to be added to the policy pages.
For example: the section on "confusing usernames". This was added to avoid people suggesting they were a bot account if they weren't a bot account, or were an admin if they weren't an admin, or to prevent impersonation.
So, if you register as "kjwenflkjclnaksdnalmsd" that's confusing, but it's not against the policy. Except a lot of people reporting usernames hadn't bothered to read the policy, and so they were just reporting names like that as confusing. For sometime people using their real names in a non-latin script were being blocked because their name was "confusing". This again led to changes in policy.
What WP really needs to do (and what they've actually done) is have a bot that checks usernames and places them on a list with descriptions of the problems, and warnings about why it might not be a problem. (There are differences between "WhitePower88" and "MartyJenkins88") -- and then have people checking the list.