If you're seriously asking, I'm happy to answer my two cents, but i'm very concerned that you don't have a sense for the reasons people would want to use G+ gated products without being part of G+.
As someone who has a personal G+ account and doesn't mind, and who runs a company that pays Google for apps for my ___domain on which we manage our email, I still had to register for G+ to be able to use the hangouts product in invitations and threads I'm having with my work email. Now that I've done so (without adding a profile photo or anything else), i'm getting tons of people adding me to circles, etc., which I don't want. It's essentially forcing a social-connection construct on a relationship with an empty profile that I don't want to create or have to maintain.
well to be fair, plenty of google employees are/have-been fully aware of (and vocal about) the various problems with forced G+ integrations on users. It's just that those employees haven't had any power to change those decisions. With the recent changes in leadership, things may change.
As someone who has a personal G+ account and doesn't mind, and who runs a company that pays Google for apps for my ___domain on which we manage our email, I still had to register for G+ to be able to use the hangouts product in invitations and threads I'm having with my work email. Now that I've done so (without adding a profile photo or anything else), i'm getting tons of people adding me to circles, etc., which I don't want. It's essentially forcing a social-connection construct on a relationship with an empty profile that I don't want to create or have to maintain.