Wasn't this a World of Warcraft security thing?

I've personally never been account hacked over 5 years on accounts I made (Yeah...One acc that me and this girl who definitely quit but kept in contact with me got hacked, but I didn't make the account so I can't vouch for security) so I can't say it's "necessary."

I've never had ANY account get hacked in all my years of playing online games, but I'd still take the extra security to make sure I have an extra layer of protection. Never been hacked in WoW, yet I still got an Authenticator for that extra layer. I'd instantly do the same if NexonNA introduced the system to GMS.

For authenticators, is there a limit of one per account? If you lost it would you be screwed forever or could you have it enabled on both a keychain and your cell phone? Is it run through a third party website or would Nexon be in charge of them still?

Accounts can only have 1, but 1 can be used on multiple accounts, assuming the accounts are linked together somehow [All Blizzard games are linked to a Battle.net account].
Losing it will pose a major problem since in order to "remove" it from said account you need to log into your account by entering the 6-8+ digit code and then entering the serial number of the Auth. No Auth, no way to get the 6-8+ code to log in in the first place. You'd have to call customer support to get it manually removed from your account and knowing Nexon, that'd be a major headache.
The authentication servers are controlled by Blizzard themselves, so I'd have to believe it'd be the same way for NexonJP.