Now imagine you have a class of 30 6-to-11-year-olds, all with unique numbers, all waiting to log into the ICT Suite. It's their first lesson, you're the teacher... see how much you get done.
You can't even GUESS at their username, because it's numeric. They won't remember it, even months later. You will have to print little cards with their usernames on, which they will be unable to read until years after they've started (BY LAW!) using computers in a network environment so the teacher has to go around one-by-one logging them in.
Not saying it's insurmountable (it's not - for a start, I push for dongle-logins because it's just easier even if they lose them in the first five minutes), but it's a problem.
Hence why humans memorise alphabetised mnemonics (i.e. a username based on their name) and let the computer convert it to a unique identifier (i.e. user-SID or equivalent).
The only place that gives me a numeric username is the UK's Government Gateway (which also needs several highly-secure passwords and security procedures to access because it lets you do everything from file your taxes to renew your driving licence). And that's been phased out because they get so much hassle with people forgetting their logins and they're planning to tie it into email addresses or social network accounts or similar.
We use usernames for a reason. If I wanted to use numbers, I'd just allocate them a login-dongle of some kind. Good luck tracking down user123127854738's history on all your systems, even if you *do* implement logging and searching. And username auditing (is this account still in use?), and lots of other boring admin tasks which are solved by using some variation of real name even despite the inherent namespace problem.