Comment 5 for bug 677929

Milan, first of all, as I can see the "adduser" command has a switch "--force-badname" that can be used in this case.
But, if you use this option every kind of strange character will be used. In other words, if you want to use the dot, you need to pass this option only when dot(s) appear in the username, and not for the rest.

Regarding Ubuntu installer, it's not something that happened to me. I just came accross this issue on a usenet newsgroup.
A person was telling that the installation was stuck at the point of creating a user.
After a while he revealed that he was trying to create a user such as "dr.john" (god bless the doctors! :-) )
In other words, you can consider fixing this issue as well, because it's almost the same issue: you need to use --force-badname

Anyway, a man page from BSD says the the dot is there for compatibility with older version of this command: http://www.manpagez.com/man/8/chown/
Perhaps somebody understood that using a dot within a username it was a very common need for the users (in facts, for instance, the dot is very common in the e-mails: name.surname@domain).
In other words I changed my idea and you're right: you can fix the tool and allow *only* dots.... otherwise, do it the other way: allow only "_" and "-"
It's your choice :-)