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.