the username is required as it is used to end the url to the users profile 

i´m also interested in this, would be a really interesting change since it´s a bit strange for users having to put a username if you don´t want to use it anymore... any ideas?
thanks!

to remove this would take a lot of custom coding mods as like i said the end of the url to the users profile is the username eg www.youriste.com/user/(username) 

Kieron is right, most of the plugins use the username in their router and other plugin parts, thus to remove this profile question, you will need to modify each plugin which uses it and replace the username variable with the different value.

Would it be possible to attribute a unique username automatically instead of asking for users to complete that question ?

It would create a unique url as well then.

Edit: something like md5(keychain)xPASSWORD(another keychain)-(blahblahblah)+encoded timestamp

I haven't tried this, but the username accepts numbers and characters, so why not change "User Name" to read "Email Address"? You should be able to do this via Languages in Admin.

That would mean the user would have to enter his/her email address twice, but we're often asked to do this anyway. Smart people will quickly learn that they can enter "xxx" but the average Joe will do what he's told.

Can't see why it wouldn't work.

You can't because of the '.' and the '@' are not authorized in the username (only numbers and characters as you spotted).

I just modified line 299 in ow_system_plugins/controllers/join.php replacing $joinData['username'].

I also defined username as none required in mysql ow_base_question and 0 (non visible) to all pages.