Don't take this personally, but I've long held (40 years or so) that most computer users don't know how to use their computers. They are enslaved by their operating system (thank you Microsoft and Apple) where the user can point, click, use application programs, and develop myriads of bad computing habits along the way, but are often unable so solve what should be rudimentary issues that invariably arise.
These bad computing habits transfer with the user when they try a different operating system and are very frustrating for the user. People who use computers would be well served by learning how to use computers, but Microsoft and Apple have made it increasingly difficult over the years to do meaningful work from the shell from which the power of the operating system, shell, and languages can be unleashed.
I would prefer to see an installer that encourages users to use lowercase user names, with a brief explanation, and require the user to check a box to deviate from the convention. I understand the slight security advantage by allowing increased variability in usernames. Some financial sites require complex usernames (upper and lower cases + numbers + symbols). I'm not an MX developer and I'll be pleased whichever direction they go on this.
