I really don't want extended family and friends sending me push notifications, so I'm happy they message me on Facebook. Like a pager, I'll check for messages when I have time.
I prefer only close family and friends who are likely to have a time-sensitive issue use SMS.
There are some people I see daily but are only Snapchat buddies. That is perfect. I don't want to see their posts on Facebook, I don't want them to text me. A photo once in a while when they see something we both are interested in is more than enough.
I couldn't imagine having to somehow tweak relationships manually using only one platform. I agree with you - having multiple platforms and communities is a-okay and automatically puts people into the "circles" G+ probably has, just by virtue of what it is.
Not saying I don't agree with certain points in the summary, but I've never once desired for every contact on ever platform to all be in the same place.