Hi Calvin,
The callback solution can only work if you have control over the server hosting your app. If you use this in the client side, you are disclosing your 'super secret' server key into the client. It can work, however, it will enable any dev to use the google api on your account. (it's not 1,2,3 ready, but it is a real issue!) You will be held liable for that.
The callback system is designed to let your server do the authentication. If used correctly used you can access the users data using your server. This has a whole different set of possibilities!
Regards
Sander