Almost all of Google's products work over the web. The whole "cloud" idea is kind of their thing. Google uses a single login for almost all products now (the same one that's used for G+). Why in the world would you think that they would somehow regress and create a separate login for this that doesn't require the browser?

That said, nothing Google's done has forced people to use G+ when a browser isn't required. So, again, why would you suspect that they would do something like that?

If this comes with a client, I seriously doubt it's going to require that you log into G+, but the login you use will be the same one you use for all other Google products I'm sure (G+ included). If it has a browser interface (which I'm sure it will), then I can assure you it will use the same login that's used for G+ just like almost all of Google's web apps.