It could be. I think even sharing image files requires a Google+ account. No thanks.
Eh... *shrug* I run hot and cold on the whole "how much Google knows about me" thing. At times I get annoyed with Google (a year ago I darn near switched to iThings), then, as in the other day, I resignedly decide "What's the difference? It's not as if anybody who wants to can't find out, anyway." Re-activated Google+ and even activated Google Now. I mean: Talk about "Every move you make, every step you take": Holy smokes. (Played with it a bit and turned it off, again, until I get a handle on its data and battery load.)
Yes the video chat functionality is tied to the Google talk plugin, while SMS functions separately.
I don't have Google Talk installed on either handset or tablet, and video chat in Hangouts works fine on both.
I'm pretty sure both parties have to have Google+ accounts in order for chats to hook up in Hangouts.