It's unlikely that concurrent sessions would be any kind of threat. However, stopping them could be inconvenient and, from a security perspective, actually harmful.
Assuming an attacker gained access to an account, the simplest solution would be to change the password of said account. But what if the user could not log in to change the password because the session was already active somewhere else? The user is now locked out completely and the hacker has unlimited access.
A system where the previous session would instead be logged out might be slightly better, but not by much. The attacker can always make a bot to log in every so often and cause a denial of service to the account holder. This would likely make the user believe that there is a bug in the system rather than that their account was hijacked and so, is less likely to reset their password or take other measures.
Instead, a system where sessions can be viewed by the logged in user would be much more effective as a suspicious session could be reported and closed and the user would then know to change their credentials.
Of course one-session-at-a-time systems are useful, but mostly to prevent cheating in games etc. Combined with expiring sessions, IP verification on sessions and allowing the user to view active sessions, concurrent sessions should be perfectly safe.