Sure, that's one use of it. But also friends with smartphones but not Google+, those I communicate with by SMS or WhatsApp primarily at the moment, the majority of my contacts. It's not even that I want rid of my SMS plan, although I can definitely see the benefit of that too.

We basically want the same thing - iMessages that works on Android, iOS and desktop, including SMS. Google are brilliant at sync and I have no doubts that'll work great (especially now with synced notifications). If they added SMS and starting a conversation by phone number (selecting from contacts obviously), it'd be the perfect messaging app in my opinion.

EDIT: Hangouts on web only work with Chrome, so that rules out a lot of devices I thought were supported. Not a big deal for me, but I guess it could break your requirements depending.