So, if you're running a... time management tool, let's say it's just a todo list. Then you create another tool, a time tracker so you can log your hours on your todo items. You should force them to sign up for it in order to use it? Personally, even if the tools were made to be able to operate separately of each other, I would give the users of my first tool access to the second one and link the accounts.

G+ is just an extension of their offerings. Just because the tides of the internets have flowed against it doesn't mean that anything involving Google plus is practically rape.