A friend turned off Google+ and Hangouts was entirely shut down for him, even on the browser side. I'm not sure if that's a recent change, but deleting a Google+ profile seems to cause some unexpectedly bad things to happen, like that.

I know for a fact it is impossible to use Hangouts on iOS devices without maintaining a Google+ profile. A friend was forcibly signed out when she deleted the profile it created for her using the iOS app. Either Google+ is on or Hangouts isn't a stable option.

Edit: To clarify, two separate friends who don't like/use Google+. Sorry if I'm being confusing when trying to add context.