You are absolutely right. I've been using GV as my general number since 2009 and I do not recall this feature not being there. It is quite handy for me when I'm overseas - not paying any roaming charges to make/receive calls - but there is nothing new or disruptive in this.
If I'm not mistaken the conversion of Talk to Hangouts last week dropped this feature, and it's only now rolling back out as part of Hangouts.
I answered multiple calls last week in a Hangout that opened itself from my Gmail when the call came in.
Interesting ... I almost never have had GMail open in a window so I didn't pay attention to this integration. Thanks for pointing this out.
This has never been the case for me, and I leave my Gmail open always. Do you have to opt in?
Just saw this checkbox in the Voice settings. It definitely wasn't there when I first started using Voice back in the beta.