I know that at least like, idk like 3-5 years ago, when I had gotten a new windows laptop (windows 7 or 8 I think), setting the main account to have the name "" (without the quotes), caused some problems with the basic functioning, including, I think, with some pre-installed programs,

So, some things were still being handled not quite right (whether that's because it shouldn't be allowed to be the username, or because programs should handle it being in the path, I'm not sure, but probably one of those.)