Login twice on same account? Good or potential security risk?

I suppose the downside is the concept of "logging in" is completely broken if two people can simultaneously log-in to the same account. It's like accessing your e-mail account from two different computers and reading a different piece of mail. I've never heard of a service that lets you do that because 1.) you're not allowed to share accounts and 2.) session log-ins will be impossible to manage if there's multiple sessions at the same time. Some game features like storage and NX is shared which could cause some conflicts.

There's probably a fundamental reason why you can log-in to two different accounts from the same computer but not the same account from two different computers.

You realize you can do the same thing with gmail (log on at two different places at the same time) since day one right?
In google docs two people can even edit the same document at the same time and see changes by the other person in real time.