Having the ability to terminate all sessions that exist for a certain account is a good idea, just as it is a good idea to have 'forget me everywhere' functionality.
A user's account should be associated with a single user. If this user wants to sign in on multiple devices, they should be able to do so. Just the same, they should be able to log themselves out on any and all devices they previously logged in, if they choose to do so.
The benefit of this is that it would log out any sessions that still exist because the user forgot to log out (which is a real possibility, mainly with shared devices).
The idea that this functionality should not be implemented because it could be misused when an account is compromised, is a non-argument: if a person gains unauthorized access to an account, they can do a whole lot of bad things. Logging out any active sessions would be one of the least damaging ones, while at the same time having a fair chance of alerting the account owner of the fact that something is wrong (which is a good thing).
(The strength of the authentication scheme (1-factor vs multi-factor, as mentioned by some other answer to this question) has little to do with this.)