It is best to keep 2-factor authentication enabled on your accounts in any case which will prevent any unauthorized access to your account. There is no reason to worry if you already have it enabled.
In extreme cases if they shut down that then all the links to other sites will be broken and I do not see any big problem in that situation. Only thing is the users will lose their informations and articles posted in Google+.
All those documents which are only linked from their original place will not be affected.
Knowledge is power.